<PAGE>
                                                                  EXHIBIT 10.27


                        FIFTH AMENDMENT TO LOAN AGREEMENT


         THIS FIFTH AMENDMENT TO 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"), and HORIZON VESSELS, INC., a Delaware corporation
("Vessels", and together with Contractors and Subsea 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 Loan Agreement dated
as of March 26, 2001, as amended by First Amendment to Loan Agreement dated as
of April 17, 2002, Second Amendment to Loan Agreement dated as of May 30, 2002,
Third Amendment to Loan Agreement dated as of August 28, 2002 and Fourth
Amendment to Loan Agreement dated as of September 30, 2002 (the "Agreement").

         B. Pursuant to the Agreement, Horizon Offshore, Inc., a Delaware
corporation ("Guarantor"), executed a Guaranty dated as of March 26, 2001 (the
"Guaranty") pursuant to which Guarantor 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 Guarantor
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.

<PAGE>

                                   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           2.00%        2.50%       3.00%     3.50%        5.00%
-------------------------------------------------------------------------------
Prime Rate Margin     -0.50%        0.00%       0.50%     1.00%        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 or equal to 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 financial statements are delivered; provided
         further, that, notwithstanding any provisions hereof the preceding 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


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         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.

         "Foreign Loan Agreement" means that certain EXIM Guaranteed Loan
Agreement among Borrowers, LLC, the Foreign Agent and the lenders referred to
therein, as amended by First Amendment to EXIM Guaranteed Loan Agreement dated
April 17, 2002, Second Amendment to EXIM Guaranteed Loan Agreement dated August
28, 2002, Third Amendment to EXIM Guaranteed Loan Agreement dated September 30,
2002 and Fourth Amendment to EXIM Guaranteed Loan Agreement dated March 6, 2003,
and 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 Guarantor and its
Subsidiaries, the consolidated net income (or loss) of Guarantor 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. Section 2.9 of the Agreement
shall be amended to read in its entirety as follows:

               Section 2.9. Amendment Fee. Borrowers agree to pay to Agent for
         the Pro-Rata benefit of Lenders an amendment fee in the amount of
         $50,000.00 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 (a) on March 6, 2003, for the fiscal quarter ending December
         31, 2002, and (b) 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.

         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. Guarantor 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 Guarantor and its Subsidiaries are
         required to


                                      -3-

<PAGE>


         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
         Guarantor 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 Guarantor, 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. Guarantor 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 Guarantor, 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 to read in its entirety as follows:

               Section 9.7. Capital Expenditures. Guarantor and its Subsidiaries
         will not incur Capital Expenditures in an aggregate amount (for
         Guarantor 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 Exhibits. Exhibit "I" to the Agreement (No
Default Certificate) is amended to conform in its entirety to Annex "A" to this
Amendment.

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


                                      -4-

<PAGE>

         authorized to sign this Amendment and each of the other Loan Documents
         to which such Borrower is or is to be a party together with specimen
         signatures of such officers.

               (b) Amendment Fee. The amendment fee referred to in Section 2.9
         for the quarter ending December 31, 2002 in the amount of $50,000.00.

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

         Section III.2. Additional Conditions. The effectiveness of this
Amendment is also subject to the satisfaction of the additional conditions
precedent that (a) the representations and warranties contained herein and in
all other Loan Documents, as amended hereby, shall be true and correct as of the
date hereof as if made on the date hereof, (b) all proceedings, corporate or
otherwise, taken in connection with the transactions contemplated by this
Amendment and all documents, instruments and other legal matters incident
thereto shall be satisfactory to Lenders, and (c) no Event of Default or
Unmatured Event of Default shall have occurred and be continuing.

                                   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
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) no Event of
Default or Unmatured Event of Default has occurred and is continuing, (d)
Borrowers are 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.


                                      -5-

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                                   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
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.


                                      -6-

<PAGE>

         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



                                   HORIZON SUBSEA SERVICES, INC.


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


                                      -7-

<PAGE>
                                   HORIZON VESSELS, INC.


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


                                      -8-

<PAGE>
                                   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



                                   BANK OF SCOTLAND


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


                                      -9-

<PAGE>

                                   HIBERNIA NATIONAL BANK


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



                                   COASTAL BANC SSB


                                   By:
                                      -----------------------------------------
                                      Michael Turner
                                      Executive Vice President


                                      -10-

<PAGE>

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


                                   HORIZON OFFSHORE, INC.


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


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

                                   Additional 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


                                      -11-

<PAGE>

                                   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


                                   HORIZON VESSELS INTERNATIONAL LTD.


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


                                   PT HORIZON INDONESIA


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


                                      -12-

<PAGE>

                                   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


                                   HORIZON OFFSHORE CONTRACTORS
                                   (MAURITIUS) LTD.


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


                                   HORIZON MARINE CONSTRUCTION
                                   (MAURITIUS) LTD.


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


                                      -13-

<PAGE>

                                   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


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


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


                                      -14-

<PAGE>

                                 LIST OF ANNEXES


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

         A                           No Default Certificate


                                      -15-

<PAGE>

                                    ANNEX "A"

                             No Default Certificate


                                      -16-