<PAGE>

                                                                  Exhibit 10.04

                                                                 EXECUTION COPY

                      AMENDED AND RESTATED CREDIT AGREEMENT


                           dated as of April 24, 2001


                                      among


                    GREAT LAKES DREDGE AND DOCK CORPORATION,
                                AS THE BORROWER,

                             THE OTHER LOAN PARTIES,
                                AS LOAN PARTIES,


                    THE FINANCIAL INSTITUTIONS PARTY HERETO,
                                 AS THE LENDERS,


                                       AND


                             BANK OF AMERICA, N.A.,
                 AS ISSUING LENDER AND THE ADMINISTRATIVE AGENT





                         BANC OF AMERICA SECURITIES LLC,
                        AS LEAD ARRANGER AND BOOK MANAGER




<PAGE>

                      AMENDED AND RESTATED CREDIT AGREEMENT


                  THIS AMENDED AND RESTATED CREDIT AGREEMENT ("AGREEMENT") is
being executed and delivered as of April 24, 2001, by and among Great Lakes
Dredge & Dock Corporation, a Delaware corporation (the "BORROWER"), the other
"Loan Parties" from time to time party to the Credit Agreement referred to and
defined below (collectively, the "LOAN PARTIES"), the financial institutions
from time to time party to such Credit Agreement referred to and defined below
(collectively, the "LENDERS") and Bank of America, N.A. (as successor to Bank of
America National Trust and Savings Association), as representative of the
Lenders (in such capacity, the "ISSUING LENDER" and the "ADMINISTRATIVE AGENT").
Undefined Capitalized terms used herein shall have the meanings ascribed to such
terms in such Credit Agreement.

                              W I T N E S S E T H:

                  WHEREAS, the Borrower, the other Loan Parties, the Lenders,
the Administrative Agent and the Issuing Lender have entered into that certain
Credit Agreement dated as of August 19, 1998, as heretofore amended by that
certain Amendment No. 1 dated as of October 8, 1999 and that certain Amendment
No.2 dated as of October 23, 2000 (the "CREDIT AGREEMENT"), pursuant to which,
among other things, the Lenders have agreed to provide, subject to the terms and
conditions contained therein, certain loans and other financial accommodations
to the Borrower;

                  WHEREAS, the Borrower has informed the Administrative Agent
and the Lenders that it has entered into a certain Stock Purchase Agreement
dated as of April 16, 2001, together with exhibits and schedules (as amended
from time to time in accordance with this Agreement, the "ACQUISITION
AGREEMENT"), pursuant to which Great Lakes/North American Site Developers, Inc.,
a newly organized wholly-owned Massachusetts direct corporate subsidiary of the
Borrower ("NASDI NEWCO"), would purchase all of the issued and outstanding
capital stock of North American Site Developers, Inc., a Massachusetts
corporation ("NASDI"), in consideration to the former NASDI shareholders of
$35,000,000 in cash, $3,000,000 in aggregate initial principal amount of junior
subordinated promissory notes issued by Newco and guaranteed by the Borrower on
a junior subordinated basis pursuant to the forms of promissory notes and
guaranty attached to the Acquisition Agreement (such promissory note being
hereinafter referred to as the "NASDI ACQUISITION NOTE"), and nonvoting capital
stock of Newco representing 20% of the issued and outstanding equity capital of
NASDI Newco;

                  WHEREAS, the Borrower has informed the Administrative Agent
and the Lenders that it desires to (i) issue and sell an additional $40,000,000
of its 11.25% Senior Subordinated Notes due 2008 under the existing Note
Indenture (the "2001 NOTE ISSUANCE") as described in the Confidential Offering
Circular dated as of April 18, 2001 (the "OFFERING CIRCULAR"), to lend
approximately $32,000,000 to NASDI Newco to facilitate NASDI Newco's payment of
the cash consideration due the former NASDI shareholders pursuant to the
Acquisition Agreement and related fees and expenses, and cause NASDI Newco to
execute and deliver to the Borrower a promissory note to evidence such loan (the
"NASDI INTERCOMPANY NOTE"), and (ii) apply the balance of the proceeds of the
2001 Note Issuance to repay a portion of the outstanding principal balance of
the Revolving Loans and to pay fees and expenses incurred

<PAGE>

by the Borrower in connection with the Acquisition Agreement, the 2001 Note
Issuance and this Agreement, and the transactions contemplated thereby;

                  WHEREAS, immediately following the consummation of the stock
purchase contemplated by the Acquisition Agreement, NASDI Newco would merge with
and into NASDI pursuant to that certain Agreement and Plan of Merger dated as of
April 24, 2001 (the "MERGER AGREEMENT"), resulting in the former management
NASDI shareholders directly owning 20% of the outstanding equity of NASDI, the
Borrower directly owning 80% of such outstanding equity and all of the issued
and outstanding Voting Stock of NASDI, and NASDI assuming by operation of law
all of NASDI Newco's liabilities, including, without limitation, its obligations
with respect to the NASDI Acquisition Note and the NASDI Intercompany Note (the
transactions contemplated by the Acquisition Agreement and the Merger Agreement
being hereinafter collectively refereed to as the "ACQUISITION");

                  WHEREAS, subject to the terms and conditions of this
Agreement, the Lenders have agreed to amend the Credit Agreement to permit the
Acquisition, the 2001 Note Issuance and the related transactions described
above; and

                  WHEREAS, in anticipation of the modifications to be effected
pursuant to this Agreement, the Lenders have agreed to reallocate their
respective Commitments and interests in the outstanding Loans and other
outstanding financial accommodations under the Credit Agreement, and to sell and
assign certain portions thereof to certain new Lenders, all pursuant to a
certain Assignment and Acceptance Agreement of even date herewith among such
Lenders, the Administrative Agent and the Borrower (the "2001 ASSIGNMENT
AGREEMENT"), which agreement would become effective upon the satisfaction of the
conditions to effectiveness of this Agreement, but immediately prior to the
effectiveness of this Agreement.

                  NOW, THEREFORE, in consideration of the foregoing premises,
the terms and conditions stated herein and other valuable consideration, the
receipt and sufficiency of which are hereby acknowledged by the Borrower, the
other Loan Parties, the Lenders and the Administrative Agent, such parties
hereby agree as follows:

                  1. AMENDMENT TO CREDIT AGREEMENT. Subject to the satisfaction
of each of the conditions set forth in PARAGRAPH 2 of this Agreement, the Credit
Agreement is amended as follows (unless otherwise specified, section, exhibit
and schedule references refer to sections, exhibits and schedules of the Credit
Agreement):

                  (a) The last sentence of SECTION 2.8.1 is deleted in its
entirety and replaced with the following provision:

         No mandatory or voluntary prepayment of principal of the Loans shall
         cause a reduction in the Revolving Commitment Amount, except as
         provided in SECTIONS 2.2 and 2.8.2.

                  (b) SECTION 2.8.2 is amended to add the following provision to
the end of such section:

                                       2

<PAGE>

         To the extent such mandatory prepayments are applied to the outstanding
         principal balance of the Revolving Loans pursuant to this section (or
         amounts applied to cash collateralize Letter of Credit Obligations
         pursuant to this section are applied to satisfy such Letter of Credit
         Obligations), the Revolving Commitment shall be thereupon permanently
         reduced by the amounts so applied.

                  (c) SECTION 6.1(c) is amended to delete the word "and" at the
end of CLAUSE (H) of such section, to redesignate existing CLAUSE (I) as new
CLAUSE (J) thereof, and to add the following new CLAUSE (I) to such section
immediately following existing CLAUSE (H):

                  (I) transactional contractors pollution coverage of not less
          than $20,000,000; and

                  (d) SECTION 6.1(p) is amended to add the following
parenthetical to such section immediately following the initial phrase "Upon a
Person becoming, after the Closing Date, a Subsidiary of the Borrower":

         (other than NASDI Newco)

                  (e) SECTION 6.1(p) is further amended to delete CLAUSE (i)
thereof in its entirety and to replace such clause with the following provision:

         (i) such Person shall execute a Subsidiary Guaranty and other
         Collateral Documents in substantially the same forms as the other
         Subsidiary Guaranties and Collateral Documents then in existence and
         otherwise in form and substance reasonably satisfactory to the
         Administrative Agent (but only to the extent not prohibited by the
         Bonding Agreement or the Intercreditor Agreement, each as in effect on
         the Closing Date)

                  (f) SECTION 6.2(a) is amended to delete the word "and" which
appears at the end of CLAUSE (i) of such section, to replace the period at the
end of CLAUSE (ii) of such section with ", and" and to add the following new
CLAUSE (iii) to the end of such section:

                  (iii)  the NASDI Acquisition and NASDI Merger.

                  (g) SECTION 6.2(b) is amended to delete CLAUSE (xi) in its
entirety and to replace such clause with the following provisions:

                  (xi) (A) the initial $5,000,000 capitalization of NASDI Newco
         and (B) an approximate $32,000,000 loan by the Borrower to NASDI Newco
         to facilitate the payment of the cash consideration due to NASDI's
         former shareholders in connection with the NASDI Acquisition, provided
         that such loan is evidenced by the NASDI Intercompany Note and pledged
         to the Administrative Agent pursuant to the Note Pledge Agreement; and

                  (h) CLAUSE (iv) of SECTION 6.2(c) is deleted in its entirety
(but not the PROVISO which immediately follows such clause) and replaced with
the following provision:

                                       3

<PAGE>

          (iv) any Subsidiary of the Borrower may make Restricted Payments
          ratably among all of its equity holders

                  (i) SECTION 6.2(f) is amended to add the following new CLAUSE
(vi) to such section immediately following existing CLAUSE (v) thereof, and to
redesignate existing CLAUSE (vi) thereof as new CLAUSE (vii):

         (vi) a Guaranty by the Borrower of the indebtedness evidenced by the
         NASDI Acquisition Note pursuant to the NASDI Acquisition Note Guaranty

                  (j) SECTION 6.2(g) is amended to delete CLAUSE (iii) of such
section in its entirety and to replace such clause with the following provision:

         (iii) is a transfer by NASDI Newco of 20% of its capital stock in the
         form of nonvoting common stock to the former management stockholders of
         NASDI as part of its payment of the purchase price consideration
         pursuant to NASDI Acquisition Agreements.

                  (k) SECTION 6.2(i) is amended to delete the phrase
"subordinated to the Obligations in a manner reasonably satisfactory to the
Administrative Agent and" set forth in CLAUSE (iv) of such section.

                  (l) SECTION 6.2(i) is further amended to delete CLAUSE (x) in
its entirety and to replace such clause with the following provision:

                  (x)  Debt evidenced by the NASDI Acquisition Note;

                  (m) SECTION 6.2(i) is further amended to delete the figure
"$115,000,000" set forth in CLAUSE (xi) of such section and to replace such
figure with the figure "$155,000,000."

                  (n) SECTION 6.2(i) is further amended to delete the cross
reference to CLAUSE (vi) of SECTION 6.2(f) set forth in CLAUSE (xiv) of such
section and to replace such cross reference with a cross reference to CLAUSE
(vii) of SECTION 6.2(f).

                  (o) SECTION 6.2(k) is amended to add the following provision
to the end of CLAUSE (i) of such section:

         , or the commercial and industrial demolition business

                  (p) SECTION 6.2(o) is amended to redesignate CLAUSE (a)
thereof as CLAUSE (i), to delete CLAUSE (b) thereof in its entirety, and to
replace such CLAUSE (b) with the following provisions:

         (ii) the Merger Agreement, the Shareholders Agreement, any of the NASDI
         Acquisition Agreements (other than amendments, supplements or other
         modifications of the NASDI Acquisition Agreements which are not adverse
         to the rights or interests of the Secured Parties under the Loan
         Documents), or the Note Indenture (other than modifications of the Note
         Indenture pursuant to the terms of Section 9.01 thereof). In addition,
         the

                                       4

<PAGE>

          Borrower shall not consent to the transfer or assignment of the NASDI
          Acquisition Note or any interest therein by any payee thereunder
          without first providing written notice to the Administrative Agent of
          the date of such transfer or assignment and the name(s) and
          address(es) of the transferee(s) or assignee(s) with respect thereto.

                  (q) SECTIONS 6.3(a) through 6.3(d) are deleted in their
entirety and replaced with the following provisions:

                      (a) CAPITAL EXPENDITURES AND PERMITTED BUSINESS
          ACQUISITIONS. The Borrower and its consolidated Subsidiaries shall not
          make or permit Capital Expenditures and Permitted Business
          Acquisitions in an aggregate amount in excess of $18,000,000
          (excluding all expenditures incurred to consummate the NASDI
          Acquisition) during any Fiscal Year commencing with Fiscal Year 2001
          (with respect to any such Fiscal Year, the "BASE CAPITAL EXPENDITURE
          AMOUNT"); PROVIDED, HOWEVER, that the Base Capital Expenditure Amount
          for any Fiscal Year after Fiscal Year 2001 may be increased by an
          amount equal to the lesser of (i) $6,000,000 and (ii) the excess, if
          any, of (A) the Base Capital Expenditure Amount for the immediately
          preceding Fiscal Year, over (B) the actual amount of Capital
          Expenditures and Permitted Business Acquisitions made by the Borrower
          and its Subsidiaries during such immediately preceding Fiscal Year.

                      (b) MAXIMUM TOTAL LEVERAGE. The Borrower and its
          consolidated Subsidiaries shall not permit the ratio (the "TOTAL
          LEVERAGE RATIO") of (i) the aggregate unpaid principal amount of Total
          Funded Debt as of the last day of any Fiscal Quarter ending during the
          periods described below (each, a "TOTAL LEVERAGE RATIO TEST DATE") to
          (ii) EBITDA for the four (4) consecutive Fiscal Quarter period ending
          as of such Total Leverage Ratio Test Date, to exceed the corresponding
          ratio set forth below opposite such period:

<TABLE>
<CAPTION>
                          Period                               Ratio
                          ------                               -----
                <S>                                         <C>
                Closing Date through and
                including December 31, 2002                 4.50 to 1.00

                January 1, 2003 through and
                including December 31, 2003                 4.25 to 1.00

                January 1, 2004 and thereafter              4.00 to 1.00
</TABLE>

                      (c) MAXIMUM SENIOR LEVERAGE. The Borrower and its
          consolidated Subsidiaries shall not permit the ratio of (i) the
          aggregate unpaid principal amount of Senior Debt as of last day of any
          Fiscal Quarter ending during the periods described below to (ii)
          EBITDA for the four (4) consecutive Fiscal Quarter period ending as of
          such date, to exceed the corresponding ratio set forth below opposite
          such period:

<TABLE>
<CAPTION>
                          Period                               Ratio
                          ------                               -----
                <S>                                         <C>
                Closing Date through and

                                       5

<PAGE>

                including December 31, 2001                 2.00 to 1.00

                January 1, 2002 through and
                including December 31, 2002                 1.75 to 1.00

                January 1, 2003 and thereafter              1.50 to 1.00
</TABLE>

                      (d) DEBT SERVICE COVERAGE RATIO. The Borrower and its
          consolidated Subsidiaries shall not permit the ratio of (i) EBITDA for
          any four (4) consecutive Fiscal Quarter period ending as of the last
          day of any Fiscal Quarter ending during the period described below to
          (ii) the sum of scheduled payments of principal (excluding mandatory
          and voluntary prepayments) with respect to Debt (including Capitalized
          Rentals attributable the principal portion of Capitalized Leases),
          plus Interest Expense, in each case for the four (4) consecutive
          Fiscal Quarter period ending as of such date, to be less than the
          corresponding ratio set forth below opposite such period:

<TABLE>
<CAPTION>
                          Period                               Ratio
                          ------                               -----
                <S>                                         <C>
                Closing Date through and
                including December 31, 2001                 1.25 to 1.00

                January 1, 2002 through and
                including December 31, 2002                 1.50 to 1.00

                January 1, 2003 through and
                including December 31, 2003                 1.75 to 1.00

                January 1, 2004 and thereafter              2.00 to 1.00.
</TABLE>

                  (r) SECTION 6.3 is further amended to add the following
provisions to the end of such section:

                      (f) INTEREST COVERAGE RATIO. The Borrower and its
          consolidated Subsidiaries shall not permit the ratio of (i) EBITDA for
          the four (4) consecutive Fiscal Quarter period ending as of March 31,
          2001 to (ii) Interest Expense for such four (4) consecutive Fiscal
          Quarter period, to be less than the 2.00 to 1.00.

                      (g) PRE-ACQUISITION EBITDA CALCULATIONS. Notwithstanding
          anything herein to the contrary, each calculation of EBITDA for the
          four (4) consecutive Fiscal Quarter periods ending June 30, September
          30 and December 31, 2001 and March 31, 2002 pursuant to SECTIONS
          6.3(b), 6.3(c) and 6.3(d) shall be determined on a combined historical
          basis for the Borrower and NASDI, shall exclude, in the case of
          NASDI's EBITDA, any deduction from such EBITDA for bonuses paid by
          NASDI during such periods (but prior to the consummation of the NASDI
          Acquisition), and otherwise shall be determined in accordance with
          GAAP.

                                       6

<PAGE>

                  (s) SECTION 7.1(d) is amended to add the following provision
to the end of such section:

          ; or an "Event of Default" shall have occurred under and as defined in
          the NASDI Acquisition Note.

                  (t) SECTION 7.1(j)(ii) is deleted in its entirety and replaced
with the following provision:

                  (ii) the failure of the Borrower (A) to own (directly or
          indirectly), free and clear of all Liens or other encumbrances (other
          than any Lien or encumbrance created by the Loan Documents), 100% of
          the outstanding shares of each class of capital stock of any
          Subsidiary Guarantor on a fully diluted basis, or, in the case of
          NASDI, at least 75% of the outstanding equity capital, or 100% of the
          outstanding Voting Stock, of NASDI, or (B) to have the power (directly
          or indirectly) to direct or cause the direction of the management or
          policies of any such Subsidiary Guarantor.

                  (u) SECTION 9.1(a) is deleted in its entirety and replaced
with the following provision:

          (a)  [intentionally omitted].

                  (v) SECTION 9.1 is further amended to add the following
provision to the end of such section:

          , and no amendment, waiver or consent shall effect an increase in any
          of the Commitments of any Lender without such Lender's express written
          approval.

                  (w) EXHIBIT F is deleted in its entirety and replaced with the
form attached hereto as ANNEX 1.

                  (x) SCHEDULE I is amended to delete the definition(s) of
"APPLICABLE BASE RATE MARGIN," "APPLICABLE COMMITMENT FEE PERCENTAGE,"
"APPLICABLE EURODOLLAR RATE MARGIN," "APPLICABLE FINANCIAL LETTER OF CREDIT FEE
PERCENTAGE," and "APPLICABLE PERFORMANCE LETTER OF CREDIT FEE PERCENTAGE," in
their entirety and to replace such definition(s) with the following provisions:

                      "APPLICABLE BASE RATE MARGIN," "APPLICABLE COMMITMENT FEE
          PERCENTAGE," "APPLICABLE EURODOLLAR RATE MARGIN," "APPLICABLE
          FINANCIAL LETTER OF CREDIT FEE PERCENTAGE," and "APPLICABLE
          PERFORMANCE LETTER OF CREDIT FEE PERCENTAGE" respectively mean, during
          any Pricing Period, the amount set forth below for such Applicable
          Base Rate Margin, Applicable Commitment Fee Percentage, Applicable
          Eurodollar Rate Margin, Applicable Financial Letter of Credit Fee
          Percentage or Applicable Performance Letter of Credit Fee Percentage,
          as the case may be, depending upon the Total Leverage Ratio as of the
          last day of the Fiscal Quarter most recently ended prior to the first
          day of such Pricing Period (calculating EBITDA in the manner described
          in SECTION 6.3(g) for the periods ending June 30, September 30 and
          December 31, 2001 and March 31, 2002):

                                       7

<PAGE>

<TABLE>
<CAPTION>
          -----------------------------------------------------------------------------------------------------------
                                                                                   Applicable         Applicable
               Total        Applicable       Applicable         Applicable         Financial         Performance
              Leverage       Base Rate       Commitment      Eurodollar Rate    Letter of Credit   Letter of Credit
               Ratio           Margin      Fee Percentage         Margin          Fee Percentage     Fee Percentage
          -----------------------------------------------------------------------------------------------------------
          <S>               <C>            <C>               <C>                <C>                <C>
          Less than or         0.00%            0.30%             1.25%              1.25%              0.625%
          equal to 2.25x
          -----------------------------------------------------------------------------------------------------------
          Greater than         0.0%             0.35%             1.50%              1.50%              0.750%
          2.25x but less
          than or equal
          to 2.75x
          -----------------------------------------------------------------------------------------------------------
          Greater than         0.25%            0.40%             1.75%              1.75%              0.875%
          2.75x but less
          than or equal
          to 3.25x
          -----------------------------------------------------------------------------------------------------------
          Greater than         0.50%            0.45%             2.00%              2.00%              1.00 %
          3.25x but less
          than or equal
          to 3.75x
          -----------------------------------------------------------------------------------------------------------
          Greater than         0.75%            0.50%             2.25%              2.25%              1.125%
          3.75x
          -----------------------------------------------------------------------------------------------------------
</TABLE>

          PROVIDED, HOWEVER, that (i) if and for so long as the Borrower shall
          have failed to timely deliver a Compliance Certificate under SECTION
          6.4(b) or SECTION 6.4(c) with respect to such Fiscal Quarter most
          recently ended, the Applicable Base Rate Margin, Applicable Commitment
          Fee Percentage, Applicable Eurodollar Rate Margin, Applicable
          Financial Letter of Credit Fee Percentage, and Applicable Performance
          Letter of Credit Fee Percentage for such Pricing Period shall be
          determined as if the Total Leverage Ratio is greater than or equal to
          3.75 to 1.00, (ii) notwithstanding the foregoing, for the period
          beginning on April 24, 2001 and ending on the last day of the first
          Pricing Period commencing after such date(I.E., pertaining to the
          Fiscal Quarter ending June 30, 2001), the Applicable Base Rate Margin,
          Applicable Commitment Fee Percentage, Applicable Eurodollar Rate
          Margin, Applicable Financial Letter of Credit Fee Percentage, and
          Applicable Performance Letter of Credit Fee Percentage for such
          Pricing Period shall be determined as if the Total Leverage Ratio is
          greater than 2.75 to 1.00 but less than or equal to 3.25 to 1.00, and
          (iii) notwithstanding the foregoing, the Applicable Performance Letter
          of Credit Fee Percentage shall be subject to change in accordance with
          SECTION 2.10(c).

                  (y) SCHEDULE I is further amended to modify the definition of
"NOTE INDENTURE" to add the phrase "and the 2001 Note Issuance" to the end of
such definition.

                  (z) SCHEDULE I is further amended to modify the definition of
"NOTE INDENTURE OBLIGATIONS" to delete the phrase "Note Indenture and Note
Issuance" set forth in CLAUSE (a) thereof and to replace such phrase with the
phrase "Note Indenture, the Note Issuance and the 2001 Note Issuance."

                                       8

<PAGE>

                  (aa) SCHEDULE I is further amended to modify the definition of
"NOTE ISSUANCE" to add the phrase "on August 19, 1998" thereto immediately
following the existing phrase "means the issuance" set forth therein.

                  (bb) SCHEDULE I is further amended to modify the definition of
"RESTRICTED PAYMENTS" to add the following provision immediately following
CLAUSE (iv) thereof and to redesignate existing CLAUSE (v) thereof as CLAUSE
(vi):

          (v) any voluntary prepayment, redemption, or repurchase or other
          voluntary payment of principal with respect to the NASDI Acquisition
          Note, or any voluntary payment or other prepayment of interest with
          respect thereto,

                  (cc) SCHEDULE I is further amended to add the following new
definitions to such schedule in their respective alphabetical or numerical
locations:

                       "2001 NOTE ISSUANCE" means the issuance by the Borrower
          on April 24, 2001 of $40,000,000 in original aggregate principal
          amount of its 11.25% Senior Subordinated Notes due 2008 pursuant to
          the Note Indenture.

                       "NASDI" means North American Site Developers, Inc., a
          Massachusetts corporation.

                       "NASDI ACQUISITION" means the Borrower's acquisition of
          80% (after giving effect to the NASDI Merger) the outstanding capital
          stock of NASDI (representing all of the issued and outstanding Voting
          Stock of NASDI) pursuant to the terms and conditions of the NASDI
          Acquisition Agreements.

                       "NASDI ACQUISITION AGREEMENTS" means, collectively, that
          certain Stock Purchase Agreement dated as of April 16, 2001 (including
          disclosure schedules) among the Borrower, NASDI Newco, NASDI and the
          former stockholders of NASDI, that certain Stockholders Agreement
          dated as of April 24, 2001 among the Borrower, NASDI Newco, NASDI and
          certain of the former stockholders of NASDI, the NASDI Merger
          Agreement, the NASDI Acquisition Note, the NASDI Intercompany Note and
          NASDI Acquisition Note Guaranty.

                       "NASDI ACQUISITION NOTE" means, collectively, those
          certain Junior Subordinated Promissory Notes dated as of April 24,
          2001 in an aggregate initial principal amount of $3,000,000 executed
          and delivered by NASDI Newco (and assumed by NASDI by operation of the
          NASDI Merger) and made payable to certain of the former non-management
          stockholders of NASDI.

                       "NASDI ACQUISITION NOTE GUARANTY" means that certain
          Subordinated Guaranty dated as of April 24, 2001 executed and
          delivered by the Borrower with respect to the NASDI Acquisition Note
          in favor of certain of the former non-management stockholders of
          NASDI.

                       "NASDI INTERCOMPANY NOTE" means that certain promissory
          note dated as of April 24, 2001 in an aggregate initial principal
          amount of approximately $32,000,000

                                       9

<PAGE>

          executed and delivered by NASDI Newco (and assumed by NASDI by
          operation of the NASDI Merger) in connection with the NASDI
          Acquisition and made payable to the Borrower.

                       "NASDI MERGER" means the merger of NASDI Newco with and
          into NASDI pursuant to the NASDI Merger Agreement.

                       "NASDI MERGER AGREEMENT" that certain Agreement and Plan
          of Merger dated as of April 24, 2001 between NASDI Newco and NASDI.

                       "NASDI NEWCO" means Great Lakes/North American Site
          Developers, Inc., a Massachusetts corporation and direct Subsidiary of
          the Borrower.

                  2. EFFECTIVENESS OF THIS AGREEMENT; CONDITIONS PRECEDENT. The
provisions of PARAGRAPH 1 of this Agreement shall be deemed to have become
effective as of the date of this Agreement, but such effectiveness shall be
expressly conditioned upon the satisfaction of each of the following:

                  (a) the Administrative Agent's receipt of originally-executed
counterparts of this Agreement executed by duly authorized officers of the
Borrower, the other Loan Parties and each of the Lenders (after giving effect to
the 2001 Assignment Agreement);

                  (b) the Administrative Agent's receipt of originally-executed
counterparts of the 2001 Assignment Agreement executed by each of the Lenders
named as parties thereto, and the Borrower, and such 2001 Assignment Agreement
shall have become effective;

                  (c) the Administrative Agent's receipt of fully-executed
copies of substituted and amended Notes executed and delivered by the Borrower
and made payable to each Lender, reflecting each Lender's reallocated Revolving
Commitment and outstanding Term Loan after giving effect to assignments and
assumptions contemplated by the 2001 Assignment Agreement;

                  (d) the Administrative Agent's receipt of originally-executed
counterparts of the Accession Agreement, Collateral Documents and each of the
other agreements, instruments, certificates, opinions and deliveries to be
executed and or delivered by NASDI, the other Loan Parties and certain other
Persons, as described in the list of closing documents attached hereto as ANNEX
2 (the "CLOSING LIST");

                  (e) the 2001 Note Issuance shall be consummated (subject to
the effectiveness of this Agreement) in accordance with the Note Indenture, and
with the Offering Circular, and the Borrower shall have received Net Cash
Proceeds of not less than $35,000,000 in connection therewith;

                  (f) (1) the Borrower shall have funded the approximate
$32,000,000 loan to be evidenced by the NASDI Intercompany Note, (2) the
Acquisition shall have been consummated, and the NASDI Acquisition Note issued,
pursuant to the terms and conditions of the Acquisition Agreement and the form
of NASDI Acquisition Note, (3) the cash and non-cash portions of the purchase
price consideration of the Acquisition shall have been paid and delivered to
NASDI's former shareholders, (4) the sum of the gross consideration and related

                                      10

<PAGE>

fees and expenses with respect to the Acquisition did not exceed $42,000,000,
and (5) the sum of the gross cash consideration and related fees and expenses
with respect to the Acquisition did not exceed $39,000,000; and

                  (g) the Administrative Agent shall have received, in
immediately available funds, payment in full of all fees then due and payable to
the Administrative Agent, the Lenders and Banc of America Securities LLC
pursuant to that certain fee letter agreement dated as of March 16, 2001 between
the Borrower and Banc of America Securities, LLC.

                  3.  REPRESENTATIONS AND WARRANTIES.

                  (a) The Borrower and each other Loan Party hereby represents
and warrants that this Agreement, the Credit Agreement as amended by this
Agreement, and each of the Loan Documents described in the Closing List and to
be executed and delivered by one or more Loan Parties (together with this
Agreement and the Credit Agreement as amended hereby, collectively, the
"AMENDMENT DOCUMENTS"), constitute legal, valid and binding obligations of the
Borrower and the other Loan Parties enforceable against the Borrower and the
other Loan Parties in accordance with their terms.

                  (b) The Borrower and each other Loan Party hereby represents
and warrants that its execution, delivery and performance of this Agreement and
the other Amendment Documents have been duly authorized by all proper corporate
action, do not violate any provision of its articles or certificate of
incorporation or bylaws, will not violate any law, regulation, court order or
writ applicable to it, and will not require the approval or consent of any
governmental agency, or of any other third party under the terms of any contract
or agreement to which it or any of its Affiliates is bound (which has not been
previously obtained), including without limitation, the Note Indenture and the
Reliance Agreement.

                  (c) The Borrower and each other Loan Party hereby represents
and warrants that, both before and after giving effect to the provisions of this
Agreement, (i) no Default or Event of Default has occurred and is continuing or
will have occurred and be continuing and (ii) all of the representations and
warranties of the Borrower and each other Loan Party contained in the Credit
Agreement and in each other Loan Document (other than representations and
warranties which, in accordance with their express terms, are made only as of an
earlier specified date) are, and will be, true and correct as of the date of its
execution and delivery hereof or thereof in all material respects as though made
on and as of such date.

                  (d) (i) All material conditions precedent to, and all consents
necessary to permit, the Acquisition pursuant to the "NASDI Acquisition
Agreements" (as proposed to be defined in PARAGRAPH 1 hereof), and the 2001 Note
Issuance pursuant to the Note Indenture, have been satisfied or delivered, (ii)
no material breach of any term or provision of the NASDI Acquisition Agreements
or the Note Indenture has occurred, (iii) the Acquisition and the 2001 Note
Issuance each have been consummated (or are being consummated simultaneously
with effectiveness of the Agreement), in accordance with all Applicable Laws,
(iv) all material governmental and material third party approvals necessary in
connection with the Acquisition and the 2001 Note Issuance shall have been
obtained and are in full force and effect, and all applicable waiting periods
shall have expired without any action being taken or threatened by any
Governmental

                                      11

<PAGE>

Authority which would restrain, prevent or otherwise impose materially adverse
conditions of the Acquisition or the 2001 Note Issuance and (v) no actions,
suits or proceedings are pending or threatened with respect to the Acquisition
or the 2001 Note Issuance or any of the NASDI Acquisition Agreements which could
reasonably be expected to have a Material Adverse Effect.

                  (ii) The Offering Circular complies in all material respects
with all applicable provisions of the 1933 Act, 1934 Act, all other federal
securities laws, state securities or "Blue Sky" laws, foreign securities laws,
general corporation law and all rules and regulations thereunder.

                  4. REAFFIRMATION, RATIFICATION AND ACKNOWLEDGMENT. The
Borrower and each other Loan Party hereby (a) ratifies and reaffirms all of its
payment and performance obligations, contingent or otherwise, and each grant of
security interests and liens in favor of the Administrative Agent, under each
Loan Document to which it is a party, (b) agrees and acknowledges that such
ratification and reaffirmation is not a condition to the continued effectiveness
of such Loan Documents and (c) agrees that neither such ratification and
reaffirmation, nor the Administrative Agent's, or any Lender's solicitation of
such ratification and reaffirmation, constitutes a course of dealing giving rise
to any obligation or condition requiring a similar or any other ratification or
reaffirmation from the Borrower or such other Loan Parties with respect to any
subsequent modifications to the Credit Agreement or the other Loan Documents. As
modified hereby, the Credit Agreement is in all respects ratified and confirmed,
and the Credit Agreement as so modified by this Amendment shall be read, taken
and so construed as one and the same instrument. Each of the Loan Documents
shall remain in full force and effect and are hereby ratified and confirmed.
Neither the execution, delivery nor effectiveness of this Agreement shall
operate as a waiver of any right, power or remedy of the Administrative Agent or
the Lenders, or of any Default or Event of Default (whether or not known to the
Administrative Agent or the Lenders), under any of the Loan Documents. This
Agreement and each of the other Amendment Documents shall constitute Loan
Documents for purposes of the Credit Agreement.

                  5. REFERENCE TO AND EFFECT ON THE CREDIT AGREEMENT.

                  (a) Upon the effectiveness of this Agreement, (i) each
reference in the Credit Agreement to "this Agreement, " "hereunder," "hereof,"
"herein" or words of like import and each reference to the Credit Agreement in
each Loan Document shall mean and be a reference to the Agreement as amended and
restated hereby and (ii) the Credit Agreement is amended as set forth herein and
is hereby restated in its entirety to read as set forth in the Credit Agreement
with the amendments specified herein.

                  (b) Except as specifically amended above, all of the terms,
conditions and covenants of the Credit Agreement shall remain unaltered and in
full force and effect and are incorporated hereby by reference and shall be
binding upon the Borrower and each other Loan Party in all respects and are
hereby ratified and confirmed, and the Credit Agreement (as amended and restated
by this Agreement) is hereby ratified and confirmed in all respects.

                  (c) The execution, delivery and effectiveness of this
Agreement shall not operate as a waiver of (i) any right, power or remedy of the
Administrative Agent or any Lender

                                      12

<PAGE>

under the Credit Agreement or any of the Loan Documents or (ii) any Default or
Event of Default under the Agreement.

                  (d) Notwithstanding anything herein to the contrary, the
Borrower's Compliance Certificate with respect to its Fiscal Quarter ended March
31, 2001 shall be prepared and delivered pursuant to SECTION 6.4(d) of the
Credit Agreement without giving effect to the amendments effected by this
Agreement to SECTION 6.3 or to the form of EXHIBIT F to the Credit Agreement.

                  6. GOVERNING LAW. This Agreement shall be governed by and
construed in accordance with the laws and decisions of the State of Illinois
(including S.H.A. 735 ILCS 105/5-1, ET. SEQ., but without giving effect to any
other conflicts of law provisions).

                  7. ADMINISTRATIVE AGENT'S EXPENSES. The Borrower hereby agrees
to promptly reimburse the Administrative Agent for all of the reasonable
out-of-pocket expenses, including, without limitation, attorneys' and
paralegals' fees, it has heretofore or hereafter incurred or incurs in
connection with the preparation, negotiation and execution of this Agreement and
the other documents, agreements and instruments contemplated hereby.

                  8. COUNTERPARTS. This Agreement may be executed in
counterparts, each of which shall be an original and all of which together shall
constitute one and the same agreement among the parties.

                  9. CONSENT. Subject to the satisfaction of each of the
conditions set forth in PARAGRAPH 2 of this Agreement, and notwithstanding
anything in the Credit Agreement, this Agreement or other Loan Documents to the
contrary, the Lenders hereby consent to the continued existence of liens and
security interests in favor of Eastern Bank in substantially all of the personal
property of NASDI after the consummation of the Acquisition, PROVIDED, THAT (i)
such liens and security shall at no time after the consummation of the
Acquisition secure more than $770,000 in aggregate obligations owing to Eastern
Bank with respect to an outstanding letter of credit issued by Eastern Bank and
related fees and expenses owing to Eastern Bank in connection therewith, and no
other obligations, (ii) such obligations shall have been satisfied in full, and
such liens and security interests shall be terminated and released, on or before
June 8, 2001 and (iii) the Borrower shall have delivered to the Administrative
Agent on or before June 8, 2001 a payoff letter from Eastern Bank and lien and
security interest release and termination documentation, in form and substance
acceptable to the Administrative Agent, to evidence such satisfaction of
obligations and termination and release of liens and security interests.


                                     * * * *

                                      13

<PAGE>

     IN WITNESS WHEREOF, this Agreement has been duly executed as of the day and
year first above written.


                                     GREAT LAKES DREDGE & DOCK CORPORATION


                                     By: /s/ Deborah A. Wensel
                                         -----------------------------------
                                     Name: Deborah A. Wensel
                                     Title: Vice President and CFO


                                     GREAT LAKES DREDGE & DOCK COMPANY


                                     By: /s/ Deborah A. Wensel
                                         -----------------------------------
                                     Name: Deborah A. Wensel
                                     Title: Vice President and CFO


                                     DAWSON MARINE SERVICES COMPANY
                                     (formerly, Dawson Dredging Company)


                                     By: /s/ Deborah A. Wensel
                                         -----------------------------------
                                     Name: Deborah A. Wensel
                                     Title: Vice President and CFO


                                     FIFTY-THREE DREDGING CORPORATION


                                     By: /s/ Paul Dinquel
                                         -----------------------------------
                                     Name: Paul Dinqel
                                     Title: Vice President


                                     GREAT LAKES CARRIBEAN DREDGING, INC.


                                     By: /s/ Deborah A. Wensel
                                         -----------------------------------
                                     Name: Deborah A. Wensel
                                     Title: Vice President and CFO


                               SIGNATURE PAGE TO
                     AMENDED AND RESTATED CREDIT AGREEMENT

<PAGE>

                                     BANK OF AMERICA, N.A. (as successor to Bank
                                     of America National Trust and Savings
                                     Association), as Administrative Agent


                                     By: /s/ Kristine B. Hyde
                                         --------------------------------
                                     Name:   Kristine B. Hyde
                                            -----------------------
                                     Title:  Vice President
                                            -----------------------


                                     BANK OF AMERICA, N.A. (as successor to Bank
                                     of America National Trust and Savings
                                     Association), as a Lender


                                     By: /s/ Jennifer L. Gerdes
                                         --------------------------------
                                     Name:   Jennifer L. Gerdes
                                            -----------------------
                                     Title:  Vice President
                                            -----------------------


                                     FLEET NATIONAL BANK (as successor to Summit
                                     Bank)


                                     By: /s/ Bonnie Gershon
                                         --------------------------------
                                     Name:   Bonnie Gershon
                                            -----------------------
                                     Title:  Vice President
                                            -----------------------



                                     LASALLE BANK NATIONAL ASSOCIATION


                                     By: /s/ Drew E. Burlak
                                         --------------------------------
                                     Name:   Drew E. Burlak
                                            -----------------------
                                     Title:  Assistant Vice President
                                            -----------------------


                                     COMERICA BANK-DETROIT

                                     By: /s/ Chris Thompson
                                         --------------------------------
                                     Name:   Chris Thompson
                                            -----------------------
                                     Title:  Vice President
                                            -----------------------


                                     THE NORTHERN TRUST COMPANY


                               SIGNATURE PAGE TO
                     AMENDED AND RESTATED CREDIT AGREEMENT

<PAGE>

                                     By: /s/ Robin H. Brody
                                         --------------------------------
                                     Name:   Robin H. Brody
                                            -----------------------
                                     Title:  Vice President
                                            -----------------------


                                     FIRSTAR BANK, N.A.


                                     By: /s/ Joseph E. Long II
                                         --------------------------------
                                     Name:   Joseph E. Long II
                                            -----------------------
                                     Title:  Senior Vice President


                                     NATIONAL CITY BANK OF MICHIGAN/ILLINOIS


                                     By: /s/ Mark R. Long
                                         --------------------------------
                                     Name:   Mark R. Long
                                            -----------------------
                                     Title:  Senior Vice President
                                            -----------------------


                                     OAK BROOK BANK- OAK BROOK


                                     By: /s/ Henry Weasel
                                         --------------------------------
                                     Name:   Henry Weasel
                                            -----------------------
                                     Title:  Vice President
                                            -----------------------


                                     WELLS FARGO BANK, NA


                                     By: /s/ R. Duncan Sinclair
                                         --------------------------------
                                     Name:   R. Duncan Sinclair
                                            -----------------------
                                     Title: Vice President
                                            -----------------------

                               SIGNATURE PAGE TO
                     AMENDED AND RESTATED CREDIT AGREEMENT

<PAGE>



                                                                         ANNEX 1


                         AMENDED COMPLIANCE CERTIFICATE
                       (Exhibit F to the Credit Agreement)


                                    Attached.


<PAGE>

                                    EXHIBIT F

                         FORM OF COMPLIANCE CERTIFICATE

     I, ________________________, hereby do certify on behalf of Great Lakes
Dredge & Dock Corporation., a Delaware corporation ( the "BORROWER"), in my
capacity solely as an officer of the Borrower and not in my individual capacity,
pursuant to the Amended and Restated Credit Agreement dated as of April __,
2001[, as heretofore amended] (the "CREDIT AGREEMENT"), by and among the
Borrower, the other Loan Parties (as defined in the Credit Agreement), the
financial institutions party to the Credit Agreement (the "LENDERS"), and Bank
of America, N.A. (as successor to Bank of America National Trust and Savings
Association), in its capacity as the Issuing Lender and in its capacity as the
Administrative Agent for the Lenders, as follows:

     1. I am the duly elected, qualified and acting _______________ of the
Borrower.

     2. No Default or Event of Default has occurred and is continuing under the
Credit Agreement on the date hereof.

     3. This certificate is the "Compliance Certificate" required to be
delivered pursuant to SECTION 6.4(d) of the Credit Agreement [for the Fiscal
Quarter ending _________, ____] [for the Fiscal Year ending _________, ____].
For the fiscal period covered by this certificate, the Borrower and its
consolidated Subsidiaries have complied with each of the covenants contained in
SECTION 6.3 of the Credit Agreement.

     4. Set forth on Schedules A through E hereto are the calculations which
provide the basis for the certification in paragraph 3 above.

     Terms which are capitalized but not otherwise defined herein shall have the
meanings ascribed to such terms in the Credit Agreement.

     IN WITNESS WHEREOF, the undersigned has executed this Certificate as of the
____ day of _________, ____.



                                   GREAT LAKES DREDGE & DOCK
                                   CORPORATION


                                   By: ___________________________
                                   Title:  ______________________