<PAGE>
                                                                  EXHIBIT 10.28

               FOURTH AMENDMENT TO EXIM GUARANTEED LOAN AGREEMENT


         THIS FOURTH AMENDMENT TO EXIM GUARANTEED LOAN AGREEMENT (this
"Amendment"), dated as of March 6, 2003, is between HORIZON OFFSHORE
CONTRACTORS, INC., a Delaware corporation ("Contractors"), HORIZON SUBSEA
SERVICES, INC., a Delaware corporation ("Subsea"), HORIZON VESSELS, INC., a
Delaware corporation ("Vessels"), and HORIZEN, L.L.C., a Delaware limited
liability company ("LLC", and together with Contractors, Subsea and Vessels the
"Borrowers"), jointly and severally, each of the financial institutions which is
or may from time to time become a party to such Loan Agreement, as amended
(collectively, "Lenders", and each a "Lender"), and SOUTHWEST BANK OF TEXAS,
N.A., a national banking association, as agent (the "Agent").


                                    RECITALS:

         A. Borrowers and Lenders entered into that certain EXIM Guaranteed Loan
Agreement dated as of August 15, 2001, as amended by First Amendment to EXIM
Guaranteed Loan Agreement dated as of April 17, 2002, Second Amendment to EXIM
Guaranteed Loan Agreement dated as of August 28, 2002 and Third Amendment to
EXIM Guaranteed Loan Agreement dated as of September 30, 2002 (the "Agreement").

         B. Pursuant to the Agreement, each Borrower, Horizon Offshore, Inc., a
Delaware corporation, and Horizon Offshore Contractors, Ltd., a company
organized and existing under the laws of the Cayman Islands, each executed a
Guaranty dated as of August 15, 2001, and ECH Offshore, S. de R.L. de C.V., a
company organized and existing under the laws of Mexico, executed a Guaranty
dated as of August 15, 2001 (under Mexican law) and a Guaranty dated as of May
30, 2002 (under Texas law), pursuant to which such Persons (as defined in the
Agreement) guaranteed to Agent the payment and performance of the Obligations
(as defined in the Agreement).

         C. Pursuant to the Agreement all the other subsidiaries of Parent
executed guaranty agreements pursuant to which such subsidiaries guaranteed the
obligations of Borrowers under the Loan Agreement.

         D. Borrowers, Lenders and Agent now desire to amend the Agreement as
herein set forth.

         NOW, THEREFORE, in consideration of the premises herein contained and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

                                   ARTICLE I.

                                   Definitions

         Section I.1. Definitions. Capitalized terms used in this Amendment, to
the extent not otherwise defined herein, shall have the meanings given to such
terms in the Agreement, as amended hereby.

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                                   ARTICLE II.

                                   Amendments

         Section II.1. Amendment to Certain Definitions. (a) The definition of
each of the following terms contained in Section 1.1 of the Agreement is amended
to read in its respective entirety as follows:

         "Applicable Margin" means, for the Levels described below, the
percentage amounts set forth below.

-------------------------------------------------------------------------------
                      Level I     Level II    Level III   Level IV    Level V
-------------------------------------------------------------------------------
LIBOR Margin           1.75%        2.25%       2.75%     3.25%        5.00%
-------------------------------------------------------------------------------
Prime Rate Margin     -0.75%       -0.25%       0.25%     0.75%        2.00%
-------------------------------------------------------------------------------


               Level I applies when the Ratio of Funded Debt to Capitalization
         is less than 0.30 to 1.00.

               Level II applies when the Ratio of Funded Debt to Capitalization
         is equal to or greater than 0.30 to 1.00 but less than 0.40 to 1.00.

               Level III applies when the Ratio of Funded Debt to Capitalization
         is equal to or greater than 0.40 to 1.00 but less than 0.45 to 1.00.

               Level IV applies when the Ratio of Funded Debt to Capitalization
         is equal to or greater than 0.45 to 1.00 but less than 0.50 to 1.00.

               Level V applies when the Ratio of Funded Debt to Capitalization
         is equal to or greater than 0.50 to 1.00.

         The applicable Level shall be adjusted, to the extent applicable,
         forty-five (45) days after the end of each fiscal quarter (or, in the
         case of any change reflected by the audited financial statements
         delivered pursuant to Section 7.1(a), ninety (90) days after the end of
         each fiscal year) based on the Ratio of Funded Debt to Capitalization
         tested for the period ending on the last day of such quarter or such
         fiscal year, as applicable; provided that if Borrowers fail to deliver
         the financial statements required by Section 7.1(a) or (b), as
         applicable, or the related No Default Certificate required by Section
         7.1(d) by the forty-fifth (45th) day (or, if applicable, the 90th day)
         after the end of any quarter or any fiscal year, as applicable, Level V
         shall apply until such


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         financial statements are delivered; provided further, that,
         notwithstanding any provision hereof to the contrary, commencing March
         6, 2003, Level IV shall apply until the date on which Borrowers deliver
         to Agent financial statements pursuant to Section 7.1(a) or (b) and the
         related No Default Certificate pursuant to Section 7.1(d) which
         evidence that as of the date of such financial statements the Ratio of
         Funded Debt to EBITDA is less than or equal to 3.50 to 1.00.

         "Applicable Rate" means (a) during the period that an Advance is a
Prime Rate Advance, the sum of the Prime Rate plus the Prime Rate Margin from
time to time in effect, and (b) during the period that an Advance is a LIBOR
Advance, the sum of the LIBOR Rate plus the LIBOR Margin from time to time in
effect.

         "Domestic Loan Agreement" means that certain Loan Agreement dated March
26, 2001, among Borrowers other than LLC, the Domestic Agent and the lenders
referred to therein, as amended by First Amendment to Loan Agreement dated April
17, 2002, Second Amendment to Loan Agreement dated May 30, 2002, Third Amendment
to Loan Agreement dated as of August 28, 2002, Fourth Amendment to Loan
Agreement dated as of September 30, 2002 and Fifth Amendment to Loan Agreement
dated as of March 6, 2003, as the same may be further amended, supplemented or
modified from time to time, and any loan agreement or credit agreement executed
in restatement and replacement therefor.

         "Net Income" means, for any period, with respect to Parent and its
Subsidiaries, the consolidated net income (or loss) of Parent and its
Subsidiaries for such period, determined in accordance with GAAP applied
consistently, (a) excluding any extraordinary items during such period, and (b)
excluding write-downs on asset impairments and any other non cash charges in an
aggregate amount which does not exceed $15,000,000.00 during the fiscal year
ended December 31, 2002

         (b) The definition of the term "Vessel Purchase Event" shall be deleted
from Section 1.1 of the Agreement.

         Section II.2. Amendment to Section 2.9. (a) The title of Section 2.9 of
the Agreement is amended to read "Facility Fee, Extension Fee and Amendment
Fee".

         (b) Clause (d) contained in Section 2.9 of the Agreement shall be
re-lettered as clause (c) and is amended to read in its entirety as follows:

               (c) Borrowers agree to pay to Agent for the Pro-Rata benefit of
         Lenders an amendment fee in the amount of $75,000.00 for each fiscal
         quarter for which the Ratio of Funded Debt to EBITDA is greater than
         3.50 to 1.00. Such amendment fee shall be payable (i) on March 6, 2003,
         for the fiscal quarter ending December 31, 2002, and (ii) for each
         fiscal quarter thereafter (commencing with the quarter ending March 31,
         2003) for which the Ratio of Funded Debt to EBITDA is greater than 3.50
         to 1.00, on the date on which Borrowers deliver to Agent the No Default
         Certificate for such fiscal quarter, but in no event later than
         forty-five (45) days after the end of such fiscal quarter.


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         Section II.3. Amendment to Section 9.5. Section 9.5 of the Agreement
shall be amended to read in its entirety as follows:

                  Section 9.5. Fixed Charge Coverage Ratio. Parent and its
         Subsidiaries will maintain a Fixed Charge Coverage Ratio of not less
         than 1.50 to 1.00; provided, however, that in the event CIT amends the
         CIT Loan Agreement to provide that Parent and its Subsidiaries are
         required to maintain a Fixed Charge Coverage Ratio of 1.40 to 1.00 or
         less for at least the quarters ending March 31, 2003 and June 30, 2003,
         then Parent and its Subsidiaries shall maintain a Fixed Charge Coverage
         Ratio of not less than (a) 1.40 to 1.00 for the quarters ending March
         31, 2003 and June 30, 2003, and (b) 1.50 to 1.00 for the quarter ending
         September 30, 2003 and at all times thereafter. The Fixed Charge
         Coverage Ratio will be calculated and tested quarterly as of the last
         day of each fiscal quarter of Parent, and for purposes of calculating
         the Fixed Charge Coverage Ratio, EBITDA shall be calculated for the
         period of four fiscal quarters ended as of the date of calculation.

         Section II.4. Amendment to Section 9.6. Section 9.6 of the Agreement
shall be amended to read in its entirety as follows:

                  Section 9.6. Ratio of Funded Debt to EBITDA. Parent and its
         Subsidiaries will maintain a Ratio of Funded Debt to EBITDA of not
         greater than (a) 4.25 to 1.00 for the quarter ended December 31, 2002,
         (b) 5.95 to 1.00 for the quarter ending March 31, 2003, (c) 6.30 to
         1.00 for the quarter ending June 30, 2003 and (d) 3.50 to 1.00 for the
         quarter ending September 30, 2003 and at all times thereafter. The
         Ratio of Funded Debt to EBITDA will be calculated and tested quarterly
         as of the last day of each fiscal quarter of Parent, and for purposes
         of calculating the Ratio of Funded Debt to EBITDA, EBITDA shall be
         calculated for the period of four fiscal quarters ended as of the date
         of calculation.

         Section II.5. Amendment to Section 9.7. Section 9.7 of the Agreement
shall be amended as follows:

                  Section 9.7. Capital Expenditures. Parent and its Subsidiaries
         will not incur Capital Expenditures in an aggregate amount (for Parent
         and its Subsidiaries) which exceeds (a) $16,000,000.00 from January 1,
         2003 through June 30, 2003, or (b) $24,000,000.00 during any calendar
         year (including the year ending December 31, 2003).

         Section II.6. Amendment to Schedules and Exhibits. Exhibit "V" to the
Agreement (No Default Certificate) is amended to conform in its entirety to
Annex "A" to this Amendment.


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                                  ARTICLE III.

                              Conditions Precedent

         Section III.1. Conditions. The effectiveness of this Amendment is
subject to the receipt by Agent of the following, each in form and substance
satisfactory to Agent and Lenders:

                  (a) Certificate-Each Corporate Borrower. A certificate of the
         Secretary or another officer of each Corporate Borrower acceptable to
         Agent certifying (i) resolutions of the board of directors of each
         Corporate Borrower which authorize the execution, delivery and
         performance by such Corporate Borrower of this Amendment and the other
         Loan Documents to which such Corporate Borrower is or is to be a party,
         and (ii) the names of the officers of each Corporate Borrower
         authorized to sign this Amendment and each of the other Loan Documents
         to which such Corporate Borrower is or is to be a party together with
         specimen signatures of such officers.

                  (b) Certificate - LLC. A certificate of a Manager or another
         officer of LLC acceptable to Agent certifying (i) resolutions of the
         Members of LLC which authorize the execution, delivery and performance
         by LLC of this Amendment and the other Loan Documents to which LLC is
         or is to be a party, and (ii) the names of the Managers or other
         officers of LLC authorized to sign this Amendment and each of the other
         Loan Documents to which LLC is or is to be a party together with
         specimen signatures of such Persons.

                  (c) Amendment Fee. The amendment fee referred to in Section
         2.9 for the quarter ending December 31, 2002 in the amount of
         $75,000.00.

                  (d) Additional Information. Such additional documents,
         instruments and information as Lenders, Agent or Eximbank may request.

                                   ARTICLE IV.

                 Ratifications, Representations, and Warranties

         Section IV.1. Ratifications. The terms and provisions set forth in this
Amendment shall modify and supersede all inconsistent terms and provisions set
forth in the Agreement and except as expressly modified and superseded by this
Amendment, the terms and provisions of the Agreement are ratified and confirmed
and shall continue in full force and effect. Borrowers and Lenders agree that
the Agreement as amended hereby shall continue to be the legal, valid and
binding obligation of such Persons enforceable against such Persons in
accordance with its terms.


         Section IV.2. Representations, Warranties and Agreements. Each Borrower
hereby represents and warrants to Lenders that (a) the execution, delivery, and
performance of this Amendment and any and all other Loan Documents executed or
delivered in connection herewith have been authorized by all requisite action on


                                      -5-

<PAGE>

the part of such Borrower and will not violate the Organizational Documents of
such Borrower, (b) the representations and warranties contained in the Agreement
as amended hereby, and all other Loan Documents are true and correct on and as
of the date hereof as though made on and as of the date hereof, (c) upon the
effectiveness of this Amendment no Event of Default or Unmatured Event of
Default will exist, (d) upon the effectiveness of this Amendment Borrowers will
be in full compliance with all covenants and agreements contained in the
Agreement as amended hereby, (e) Borrowers are indebted to Lenders pursuant to
the terms of the Notes, as the same may have been renewed, modified, extended
and rearranged, (f) the liens, security interests, encumbrances and assignments
created and evidenced by the Loan Documents are, respectively, valid and
subsisting liens, security interests, encumbrances and assignments and secure
the Notes as the same may have been renewed, modified or rearranged, and (g) no
Borrower has any claims, credits, offsets, defenses or counterclaims arising
from the Loan Documents or any Lender's performance under the Loan Documents.


                                   ARTICLE V.

                                  Miscellaneous

         Section V.1. Survival of Representations and Warranties. All
representations and warranties made in this Amendment or any other Loan
Documents including any Loan Document furnished in connection with this
Amendment shall fully survive the execution and delivery of this Amendment and
the other Loan Documents, and no investigation by any Lender or any closing
shall affect the representations and warranties or the right of any Lender to
rely on them.

         Section V.2. Reference to Agreement. Each of the Loan Documents,
including the Agreement and any and all other agreements, documents, or
instruments now or hereafter executed and delivered pursuant to the terms hereof
or pursuant to the terms of the Agreement, as amended hereby, are hereby amended
so that any reference in such Loan Documents to the Agreement shall mean a
reference to the Agreement, as amended hereby.

         Section V.3. Expenses. As provided in the Agreement, Borrowers agree to
pay on demand all costs and expenses incurred by Agent in connection with the
preparation, negotiation and execution of this Amendment and the other documents
and instruments executed pursuant hereto and any and all amendments,
modifications and supplements thereto, including, without limitation, the costs
and fees of Agent's legal counsel, and all costs and expenses incurred by Agent
in connection with the enforcement or preservation of any rights under the
Agreement, as amended hereby, or any other Loan Document, including, without
limitation, the costs and fees of Agent's legal counsel.

         Section V.4. Severability. Any provision of this Amendment held by a
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

         SECTION V.5. APPLICABLE LAW. THIS AMENDMENT AND ALL OTHER LOAN
DOCUMENTS EXECUTED PURSUANT HERETO SHALL BE DEEMED TO HAVE BEEN MADE AND TO BE


                                      -6-

<PAGE>

PERFORMABLE IN HOUSTON, HARRIS COUNTY, TEXAS AND SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS.

         Section V.6. Successors and Assigns. This Amendment is binding upon and
shall inure to the benefit of Agent, Issuing Bank, each Lender and each Borrower
and their respective successors and assigns, except that no Borrower may assign
or transfer any of its rights or obligations hereunder without the prior written
consent of Agent.

         Section V.7. Counterparts. This Amendment may be executed in one or
more counterparts, each of which when so executed shall be deemed to be an
original, but all of which when taken together shall constitute one and the same
instrument.

         Section V.8. Effect of Waiver. No consent or waiver, express or
implied, by Agent, Issuing Bank or any Lender to or for any breach of or
deviation from any covenant, condition or duty by any Borrower shall be deemed a
consent or waiver to or of any other breach of the same or any other covenant,
condition or duty.

         Section V.9. Headings. The headings, captions, and arrangements used in
this Amendment are for convenience only and shall not affect the interpretation
of this Amendment.

         SECTION V.10. ENTIRE AGREEMENT. THIS AMENDMENT AND ALL OTHER
INSTRUMENTS, DOCUMENTS, AND AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH
THIS AMENDMENT EMBODY THE FINAL, ENTIRE AGREEMENT AMONG THE PARTIES HERETO WITH
RESPECT TO THE SUBJECT MATTER HEREOF AND THEREOF AND SUPERSEDE ANY AND ALL PRIOR
COMMITMENTS, AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR
ORAL, RELATING TO THIS AMENDMENT AND THE OTHER INSTRUMENTS, DOCUMENTS AND
AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH THIS AMENDMENT, AND MAY NOT
BE CONTRADICTED OR VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT
ORAL AGREEMENTS OR DISCUSSIONS OF THE PARTIES HERETO. THERE ARE NO ORAL
AGREEMENTS AMONG THE PARTIES HERETO.

         Executed as of the date first written above.


                                   BORROWERS:

                                   HORIZON OFFSHORE CONTRACTORS, INC.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Executive Vice President


                                      -7-

<PAGE>

                                   HORIZEN, L.L.C.


                                   By:
                                      ----------------------------------------
                                      R. Clay Etheridge
                                      President



                                   HORIZON SUBSEA SERVICES, INC.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Executive Vice President


                                      -8-

<PAGE>

                                   HORIZON VESSELS, INC.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Executive Vice President



                                   AGENT:

                                   SOUTHWEST BANK OF TEXAS, N.A., as Agent


                                   By:
                                      ----------------------------------------
                                      Valerie Gibbs
                                      Senior Vice President



                                   LENDERS:

                                   SOUTHWEST BANK OF TEXAS, N.A.


                                   By:
                                      ----------------------------------------
                                      Valerie Gibbs
                                      Senior Vice President



                                   DRESDNER BANK LATEINAMERIKA AG


                                   By:
                                      ----------------------------------------
                                      Maria Fernandez de Castro
                                      Marketing Officer


                                   By:
                                      ----------------------------------------
                                      Sergio Goloubeff
                                      First Vice President


                                      -9-


<PAGE>


                                   BANK OF SCOTLAND


                                   By:
                                      ----------------------------------------
                                      Joseph Fratus
                                      Vice President


                                   HIBERNIA NATIONAL BANK


                                   By:
                                      ----------------------------------------
                                      Gary Culbertson
                                      Vice President


                                      -10-

<PAGE>

         Each of the undersigned Guarantors (a) hereby consents and agrees to
this Amendment, (b) agrees that all references to the Loan Agreement contained
in the Guaranty Agreement executed by such Person shall constitute references to
such Loan Agreement as amended by this Amendment and as the same may be further
amended, and (c) agrees that the Guaranty Agreement executed by such Person
shall remain in full force and effect and shall continue to be the legal, valid
and binding obligations of such Guarantor, enforceable against such Guarantor in
accordance with its terms and shall evidence such Guarantor's guaranty of the
Notes as renewed, modified and extended from time to time.


                                   HORIZON OFFSHORE, INC.


                                   By:
                                       ----------------------------------------
                                       David W. Sharp
                                       Executive Vice President



                                   HORIZON OFFSHORE CONTRACTORS, LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director



                                   ECH OFFSHORE, S. DE R.L. DE C.V.


                                   By:
                                      ----------------------------------------
                                      R. Clay Etheridge
                                      Sole Manager


                                      -11-

<PAGE>
                                   Other Guarantors

                                   PROGRESSIVE PIPELINE CONTRACTORS, INC.
                                   AFFILIATED MARINE CONTRACTORS, INC.
                                   TEXAS OFFSHORE CONTRACTORS CORP.
                                   FLEET PIPELINE SERVICES, INC.
                                   GULF OFFSHORE CONSTRUCTION, INC.
                                   BAYOU MARINE CONTRACTORS, INC.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Executive Vice President


                                   HORIZON GROUP L.D.C.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director


                                   HORIZON OFFSHORE NIGERIA LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director


                                   TIBURON INGENIERIA Y CONSTRUCCION, S. DE
                                   R.L. DE C.V.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Secretary


                                      -12-

<PAGE>
                                   HORIZON VESSELS INTERNATIONAL LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Vice President


                                   PT HORIZON INDONESIA


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Commissioner


                                   HORIZON OFFSHORE INTERNATIONAL LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Executive Vice President


                                   HORIZON MARINE CONSTRUCTION LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Executive Vice President


                                   HORIZON OFFSHORE PTE. LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director


                                      -13-

<PAGE>

                                   HORIZON OFFSHORE CONTRACTORS
                                   (MAURITIUS) LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director


                                   HORIZON MARINE CONSTRUCTION
                                   (MAURITIUS) LTD.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director


                                   HORIZON C-BAY COSTA AFUERA, S. DE
                                   R.L. DE C.V.


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Member


                                   HOC OFFSHORE, S. DE R.L. DE C.V.


                                   By:
                                      ----------------------------------------
                                      R. Clay Etheridge
                                      President


                                   PT ARMANDI PRANAUPAYA


                                   By:
                                      ----------------------------------------
                                      David W. Sharp
                                      Director


                                      -14-

<PAGE>

                                 LIST OF ANNEXES


       Annex                            Document
       -----                            --------

         A                              No Default Certificate


                                      -15-

<PAGE>

                                    ANNEX "A"

                             No Default Certificate


                                      -16-