<PAGE>
                                                                  EXHIBIT 10.43

                        SIXTH AMENDMENT TO LOAN AGREEMENT


         THIS SIXTH AMENDMENT TO LOAN AGREEMENT (this "Amendment"), dated as of
January 30, 2004, 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, Lenders and Agent 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, Fourth Amendment
to Loan Agreement dated as of September 30, 2002 and Fifth Amendment to Loan
Agreement dated as of March 6, 2003 (the "Agreement").


         A. 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).

         B. 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.

         C. 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:


                                   1. ARTICLE

                                   Definitions

1.1. Section 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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                                   2. ARTICLE

                                   Amendments

2.1. Section Amendment to Certain Definitions. 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:

         "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, Fourth
         Amendment to EXIM Guaranteed Loan Agreement dated March 6, 2003 and
         Fifth Amendment to EXIM Guaranteed Loan Agreement dated January 30,
         2004, 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.

2.2. Section Amendment to Section 7.1. (a) Clauses (e), (f) and (h) contained in
Section 7.1 of the Agreement are amended to read in their entirety as follows:

         (e) Borrowing Base Certificate. As soon as available, and in any event
         within fifteen (15) days after the end of each month of each fiscal
         year of Borrowers, a Borrowing Base Certificate as of the last day of
         such month certified by an officer of each Borrower acceptable to
         Agent.

         (f) Weekly Accounts Receivable Reports. Beginning not later than 5:00
         p.m. on February 9, 2004, and continuing not later than 5:00 p.m. on
         each Monday thereafter, aged accounts receivable reports for each
         Borrower as of the previous Friday at 5:00 p.m., certified by an
         officer of each Borrower acceptable to Agent.

         (h) Projections. Not later than February 2, 2004, a projected balance
         sheet, income statement and statement of cash flows for Guarantor and
         its Subsidiaries for the fiscal year ending December 31, 2004 on a Base
         Case basis and a Best Case basis each, certified by an officer of
         Guarantor acceptable to Agent and reviewed by FTI Consulting, Inc.

         (b) Clauses (o), (p), (q) and (r) shall be added to Section 7.1 of the
Agreement and shall read in their entirety as follows:

         (o) Weekly Accounts Payable Reports. Beginning not later than 5:00 p.m.
         on February 9, 2004, and continuing not later than 5:00 p.m. on each
         Monday thereafter, aged accounts payable reports for each Borrower, as
         of the previous Friday at 5:00 p.m. certified by an officer of each
         Borrower acceptable to Agent.


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         (p) Weekly Cash Flow. Beginning not later than 5:00 p.m. on February 9,
         2004, and continuing not later than 5:00 p.m. on each Monday
         thereafter, cash flow reports for each Borrower, on a rolling thirteen
         (13) week basis for the thirteen (13) consecutive weeks ended as of the
         previous Friday at 5:00 p.m. certified by an officer of each Borrower
         acceptable to Agent.

         (q) Elliott Term Sheet. Not later than February 2, 2004, a Confidential
         Information Memorandum prepared by Raymond James, containing, among
         other things, terms related to the issuance of subordinated bonds in an
         amount not less than $50,000,000.00 to be purchased by Elliott
         Associates, L.P and/or other investors.

         (r) Availability Certificate. Not later than 15 days after February 16,
         2004 and March 15, 2004, a certificate of Borrower (certified by an
         officer of each Borrower acceptable to Agent), containing a calculation
         of availability under the Borrowing Base ("Availability") as of such
         February 16 and March 15 (as if such date were a date on which the
         Borrowing Base was otherwise required to be determined pursuant to this
         Agreement), and, if Availability is determined to be zero or a negative
         amount, a statement regarding the actions to be taken by Borrowers and
         the events which are expected to take place such that by the end of
         such month, Availability will be a positive number.


                                   3. ARTICLE

                              Conditions Precedent

3.1. Section 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
         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) Fees. An amendment fee payable to Agent for the Pro-Rata benefit of
         Lenders in the amount of $75,000.00, and an amendment fee payable to
         the Agent in the amount of $5,000.00.

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


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                                   4. ARTICLE

                 Ratifications, Representations, and Warranties

4.1. Section 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.

         (a) Section Representations, Warranties and Agreements. Each Borrower
         hereby represents and warrants to Lenders that 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, 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, no Event of Default or
         Unmatured Event of Default has occurred and is continuing, Borrowers
         are in full compliance with all covenants and agreements contained in
         the Agreement as amended hereby, Borrowers are indebted to Lenders
         pursuant to the terms of the Notes, as the same may have been renewed,
         modified, extended and rearranged, 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 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. ARTICLE

                                  Miscellaneous

5.1. Section 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.

5.2. Section 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


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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.

5.3. Section 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.

5.4. Section 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.

5.5. SECTION 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.

5.6. Section 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.

5.7. Section 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.

5.8. Section 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.

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

5.10. SECTION AMENDMENT TO LETTER AGREEMENTS. EFFECTIVE AS OF THE DATE OF THIS
AMENDMENT, (A) THE FIFTH PARAGRAPH CONTAINED IN THE LETTER AGREEMENT DATED
AUGUST 7, 2003 (THE "FIRST LETTER


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AGREEMENT") AND (B) THE FIFTH PARAGRAPH CONTAINED IN THE LETTER AGREEMENT DATED
NOVEMBER 10, 2003 (THE "SECOND LETTER AGREEMENT") ARE NO LONGER IN EFFECT, AND
THE FIRST LETTER AGREEMENT AND THE SECOND LETTER AGREEMENT ARE AMENDED TO DELETE
THE FIFTH PARAGRAPHS IN THEIR ENTIRETY. ALL PARTIES HERETO AGREE TO SUCH
DELETION.

5.11. SECTION 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


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                                         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 AG, NEW YORK AND
                                         GRAND CAYMAN BRANCHES


                                         By:
                                             ----------------------------------
                                         Name:
                                               --------------------------------
                                         Title:
                                               --------------------------------


                                         By:
                                             ----------------------------------
                                         Name:
                                               --------------------------------
                                         Title:
                                               --------------------------------


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                                         BANK OF SCOTLAND


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



                                         HIBERNIA NATIONAL BANK


                                         By:
                                             ----------------------------------
                                             Tammy Boyd
                                             Vice President


                                      -8-

<PAGE>

                                         COASTAL BANC SSB


                                         By:
                                             ----------------------------------
                                             Joe M. Peddy
                                             Senior Vice President




         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


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<PAGE>

         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


                                       HORIZON GROUP L.D.C.


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


                                       HORIZON OFFSHORE NIGERIA LTD.


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


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                                       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


                                       HORIZON OFFSHORE INTERNATIONAL LTD.


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


                                       HORIZON MARINE CONSTRUCTION LTD.


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


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                                       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


                                       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:
                                          -------------------------------------
                                          David W. Sharp

                                       Title:
                                              ---------------------------------


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                                       PT ARMANDI PRANAUPAYA


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


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


                                       By:
                                          -------------------------------------
                                          Bill J. Lam
                                          Sole Manager


                                       HORIZON MARINE CONTRACTORS
                                       (MALAYSIA) SDN BHD


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


                                       HORIZON OFFSHORE SERVICES, LTD.


                                       By:
                                          -------------------------------------
                                          David W. Sharp
                                          Executive Vice President and Chief
                                          Financial Officer


                                      MARINE LEASING (LABUAN) PTE LTD


                                       By:
                                          -------------------------------------
                                          David W. Sharp
                                          Executive Vice President and Chief
                                          Financial Officer


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