<Page>
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                                 UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                            ------------------------

                                   FORM 10-Q

<Table>
<C>        <S>
   /X/     QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
           SECURITIES EXCHANGE ACT OF 1934

           FOR THE QUARTERLY PERIOD ENDED SEPTEMBER 30, 2001

                                  OR

   / /     TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
           SECURITIES EXCHANGE ACT OF 1934
</Table>

                         Commission file number 0-23653

                            ------------------------

                             HORIZON OFFSHORE, INC.

             (Exact name of registrant as specified in its charter)

<Table>
<S>                                            <C>
               DELAWARE                                     76-0487309
     (State or other jurisdiction                        (I.R.S. employer
   of incorporation or organization)                   identification no.)
</Table>

                      2500 CITYWEST BOULEVARD, SUITE 2200
                              HOUSTON, TEXAS 77042
                    (Address of principal executive offices)
                                   (Zip code)

                                 (713) 361-2600
              (Registrant's telephone number, including area code)

                            ------------------------

    Indicate by check mark whether the registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. Yes /X/  No / /

    The number of shares of the registrant's common stock outstanding as of
November 1, 2001 was 23,366,361.

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
<Page>
                         PART I--FINANCIAL INFORMATION

ITEM 1. FINANCIAL STATEMENTS

                    HORIZON OFFSHORE, INC. AND SUBSIDIARIES

                          CONSOLIDATED BALANCE SHEETS

                       (IN THOUSANDS, EXCEPT SHARE DATA)

<Table>
<Caption>
                                                              SEPTEMBER 30,   DECEMBER 31,
                                                                  2001            2000
                                                              -------------   ------------
                                                               (UNAUDITED)
<S>                                                           <C>             <C>
                                          ASSETS
CURRENT ASSETS:
  Cash and cash equivalents.................................    $  3,895        $  9,243
  Accounts receivable--
    Contract receivables....................................      28,799          20,436
    Costs in excess of billings.............................      99,366          17,611
    Affiliated parties......................................          70           6,874
  Inventory.................................................       1,705           1,947
  Other current assets......................................       4,167           2,537
                                                                --------        --------
      Total current assets..................................     138,002          58,648
PROPERTY AND EQUIPMENT, net.................................     209,501         176,895
OTHER ASSETS................................................       7,100           3,882
                                                                --------        --------
                                                                $354,603        $239,425
                                                                ========        ========
                           LIABILITIES AND STOCKHOLDERS' EQUITY
CURRENT LIABILITIES:
  Accounts payable..........................................    $ 13,907        $  8,192
  Accrued liabilities.......................................       5,736           4,953
  Accrued job costs.........................................      24,959          22,484
  Billings in excess of costs...............................         535             550
  Current maturities of long-term debt......................       7,238          12,517
                                                                --------        --------
      Total current liabilities.............................      52,375          48,696
LONG-TERM DEBT, net of current maturities...................     104,242          78,415
DEFERRED INCOME TAXES.......................................      12,783           8,279
                                                                --------        --------
      Total liabilities.....................................     169,400         135,390
COMMITMENTS AND CONTINGENCIES
STOCKHOLDERS' EQUITY:
  Preferred stock, $1 par value, 5,000,000 shares
    authorized, none issued and outstanding.................          --              --
  Common stock, $1 par value, 35,000,000 shares authorized,
    24,346,480 shares issued at September 30, 2001 and
    19,869,098 at December 31, 2000.........................      13,539           9,162
  Additional paid-in capital................................     154,628          88,245
  Retained earnings.........................................      23,484          13,209
  Treasury stock, at cost, 983,076 shares at September 30,
    2001 and 1,003,400 at December 31, 2000.................      (6,448)         (6,581)
                                                                --------        --------
      Total stockholders' equity............................     185,203         104,035
                                                                --------        --------
                                                                $354,603        $239,425
                                                                ========        ========
</Table>

  The accompanying notes are an integral part of these consolidated financial
                                  statements.

                                       2
<Page>
                    HORIZON OFFSHORE, INC. AND SUBSIDIARIES

                     CONSOLIDATED STATEMENTS OF OPERATIONS

                                  (UNAUDITED)
                (IN THOUSANDS, EXCEPT SHARE AND PER SHARE DATA)

<Table>
<Caption>
                                                             THREE MONTHS ENDED         NINE MONTHS ENDED
                                                                SEPTEMBER 30,             SEPTEMBER 30,
                                                           -----------------------   -----------------------
                                                              2001         2000         2001         2000
                                                           ----------   ----------   ----------   ----------
<S>                                                        <C>          <C>          <C>          <C>
CONTRACT REVENUES........................................  $   60,965   $   56,286   $  198,369   $  108,780
COST OF CONTRACT REVENUES................................      53,283       46,022      168,179       90,848
                                                           ----------   ----------   ----------   ----------
  Gross profit...........................................       7,682       10,264       30,190       17,932
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES.............       3,022        2,427       10,127        6,808
                                                           ----------   ----------   ----------   ----------
  Operating income.......................................       4,660        7,837       20,063       11,124
OTHER:
  Interest expense.......................................      (1,317)      (2,052)      (4,375)      (5,644)
  Interest income........................................          76           80          369          232
  Gain/(loss) on sale of fixed assets....................          --            6          (22)           6
  Other income (expense).................................        (360)         (54)        (382)         (43)
                                                           ----------   ----------   ----------   ----------
NET INCOME BEFORE INCOME TAXES, EXTRAORDINARY ITEM AND
  CUMULATIVE EFFECT OF ACCOUNTING CHANGE.................       3,059        5,817       15,653        5,675
INCOME TAX PROVISION.....................................         307        2,112        4,810        2,087
                                                           ----------   ----------   ----------   ----------
NET INCOME BEFORE EXTRAORDINARY ITEM AND CUMULATIVE
  EFFECT OF ACCOUNTING CHANGE............................       2,752        3,705       10,843        3,588
EXTRAORDINARY LOSS, NET OF TAXES OF $306.................          --           --         (568)          --
CUMULATIVE EFFECT OF ACCOUNTING CHANGE, NET OF TAXES OF
  $745...................................................          --           --           --        1,381
                                                           ----------   ----------   ----------   ----------
NET INCOME...............................................  $    2,752   $    3,705   $   10,275   $    4,969
                                                           ==========   ==========   ==========   ==========
EARNINGS PER SHARE--BASIC:
  NET INCOME BEFORE EXTRAORDINARY ITEM AND CUMULATIVE
    EFFECT OF ACCOUNTING CHANGE..........................  $     0.12   $     0.20   $     0.49   $     0.19
  EXTRAORDINARY LOSS.....................................          --           --        (0.03)          --
  CUMULATIVE EFFECT OF ACCOUNTING CHANGE.................          --           --           --         0.07
                                                           ----------   ----------   ----------   ----------
  NET INCOME--BASIC......................................  $     0.12   $     0.20   $     0.46   $     0.26
                                                           ==========   ==========   ==========   ==========
EARNINGS PER SHARE--DILUTED:
  NET INCOME BEFORE EXTRAORDINARY ITEM AND CUMULATIVE
    EFFECT OF ACCOUNTING CHANGE..........................  $     0.12   $     0.19   $     0.47   $     0.19
  EXTRAORDINARY LOSS.....................................          --           --        (0.02)          --
  CUMULATIVE EFFECT OF ACCOUNTING CHANGE.................          --           --           --         0.07
                                                           ----------   ----------   ----------   ----------
  NET INCOME--DILUTED....................................  $     0.12   $     0.19   $     0.45   $     0.26
                                                           ==========   ==========   ==========   ==========
SHARES USED IN COMPUTING NET INCOME
  PER SHARE--BASIC.......................................  23,035,564   18,827,558   22,149,940   18,815,247
  PER SHARE--DILUTED.....................................  23,351,441   19,758,322   22,947,230   19,384,844
</Table>

  The accompanying notes are an integral part of these consolidated financial
                                  statements.

                                       3
<Page>
                    HORIZON OFFSHORE, INC. AND SUBSIDIARIES
                     CONSOLIDATED STATEMENTS OF CASH FLOWS
                                  (UNAUDITED)
                                 (IN THOUSANDS)

<Table>
<Caption>
                                                               NINE MONTHS ENDED
                                                                 SEPTEMBER 30,
                                                              -------------------
                                                                2001       2000
                                                              --------   --------
<S>                                                           <C>        <C>
CASH FLOWS FROM OPERATING ACTIVITIES:
  Net income before extraordinary item......................  $ 10,843   $  4,969
  Adjustments to reconcile net income to net cash provided
    by (used in) operating activities--
    Depreciation and amortization...........................     9,270      6,552
    Cumulative effect of accounting change..................        --     (2,126)
    Deferred income taxes...................................     4,810      2,831
    Loss on sale of fixed assets............................        22         --
    Expense recognized for issuance of treasury stock for
      401(k) plan contributions.............................       260        188
    Changes in operating assets and liabilities--
      Accounts receivable...................................    (1,559)   (17,190)
      Costs in excess of billings...........................   (81,755)    (7,663)
      Billings in excess of costs...........................       (15)     2,707
      Inventory.............................................       242       (778)
      Other current assets..................................    (1,152)      (482)
      Accounts payable......................................     5,715      1,931
      Accrued liabilities...................................       783      1,663
      Accrued job costs.....................................     2,475     11,142
      Due to related parties................................        --         50
                                                              --------   --------
        Net cash provided by (used in) operating
          activities........................................   (50,061)     3,794
CASH FLOWS FROM INVESTING ACTIVITIES:
  Purchases and additions to equipment......................   (38,400)   (15,947)
  Drydock costs.............................................    (4,385)    (1,546)
  Proceeds from sale of fixed assets........................         4         --
                                                              --------   --------
        Net cash used in investing activities...............   (42,781)   (17,493)
CASH FLOWS FROM FINANCING ACTIVITIES:
  Borrowings under long-term debt...........................    70,849     20,797
  Loan fees.................................................    (1,208)      (234)
  Extraordinary loss........................................      (874)        --
  Principal payments on long-term debt......................   (50,301)    (8,240)
  Stock option transactions and other.......................     3,009        162
  Issuance of common stock..................................    66,019         --
                                                              --------   --------
        Net cash provided by financing activities...........    87,494     12,485
                                                              --------   --------
NET DECREASE IN CASH AND CASH EQUIVALENTS...................    (5,348)    (1,214)
CASH AND CASH EQUIVALENTS AT BEGINNING OF PERIOD............     9,243      8,117
                                                              --------   --------
CASH AND CASH EQUIVALENTS AT END OF PERIOD..................  $  3,895   $  6,903
                                                              ========   ========
SUPPLEMENTAL DISCLOSURES:
  Cash paid for interest....................................  $  5,202   $  4,943
                                                              ========   ========
  Purchase and additions to equipment with the issuance of a
    capital lease obligation................................  $     --   $  1,300
                                                              ========   ========
  Purchase and additions to equipment with the issuance of
    common stock............................................  $  1,605   $     --
                                                              ========   ========
</Table>

  The accompanying notes are an integral part of these consolidated financial
                                  statements.

                                       4
<Page>
                    HORIZON OFFSHORE, INC. AND SUBSIDIARIES

                   NOTES TO CONSOLIDATED FINANCIAL STATEMENTS

1.  BUSINESS AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES:

    The consolidated interim financial statements included herein have been
prepared by Horizon Offshore, Inc. (a Delaware corporation) and its subsidiaries
(Horizon), and are unaudited, except for the balance sheet at December 31, 2000,
which has been prepared from the audited financial statements. In the opinion of
management, the unaudited consolidated interim financial statements include all
adjustments necessary for a fair presentation of the financial position as of
September 30, 2001, the statements of operations for each of the three and nine
month periods ended September 30, 2001 and 2000, and the statements of cash
flows for the nine months ended September 30, 2001 and 2000. Although management
believes the unaudited interim related disclosures in these consolidated interim
financial statements are adequate to make the information presented not
misleading, certain information and footnote disclosures normally included in
annual audited financial statements prepared in accordance with generally
accepted accounting principles have been condensed or omitted pursuant to the
rules of the Securities and Exchange Commission. The results of operations for
the nine months ended September 30, 2001 are not necessarily indicative of the
results to be expected for the entire year ending December 31, 2001. The
consolidated interim financial statements included herein should be read in
conjunction with the audited financial statements and notes thereto included in
our 2000 Annual Report on Form 10-K.

BUSINESS

    Horizon provides offshore construction services to the oil and gas industry.
These services generally consist of laying and burying marine pipelines for the
transportation of oil and gas, and installing and salvaging production platforms
and other marine structures. Work is performed primarily on a fixed-price or
day-rate basis, or a combination of these.

CUSTOMERS

    We have domestic operations in one industry segment, the marine construction
service industry to offshore oil and gas companies. We had one major oil and gas
customer whose revenues accounted for 52.9% of consolidated revenues for the
nine months ended September 30, 2001. Our revenues generally do not depend on
any one customer. The level of construction services required by a customer
depends on the size of their capital expenditure budget for construction for the
year. Consequently, customers that account for a significant portion of contract
revenues in one year may represent an immaterial portion of contract revenues in
subsequent years.

INTEREST CAPITALIZATION

    Interest is capitalized on the average amount of accumulated expenditures
for equipment that is undergoing major modifications and refurbishment prior to
being placed into service. Interest is capitalized using an effective rate based
on related debt until the equipment is placed into service. Interest expense for
the three and nine month periods ended September 30, 2001 is net of $403,015 and
$605,210 of capitalized interest, respectively. Interest capitalized during the
three and nine month periods ended September 30, 2000 was $135,000 and $262,000,
respectively.

INVENTORY

    Inventory consists of structures held for resale from derrick salvage
services. The inventory is reported at the lower of cost or market value.
Horizon periodically assesses the net realizable value of its inventory items.

RECENT ACCOUNTING PRONOUNCEMENTS

    In August 2001, the Financial Accounting Standards Board (FASB) issued SFAS
No. 144, "Accounting for the Impairment of Disposal of Long-Lived Assets". The
FASB's new rules on asset impairment

                                       5
<Page>
                    HORIZON OFFSHORE, INC. AND SUBSIDIARIES

             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)

supersede SFAS No. 121, "Accounting for the Impairment of Long-Lived Assets and
for Long-Lived Assets to be Disposed Of", and will be effective for the
Company's fiscal year beginning January 1, 2002. The Company is currently
evaluating the impact that adoption of this standard will have on its financial
statements.

2.  PROPERTY AND EQUIPMENT:

    Property and equipment consists of the following (in thousands):

<Table>
<Caption>
                                                              SEPTEMBER 30,   DECEMBER 31,
                                                                  2001            2000
                                                              -------------   ------------
<S>                                                           <C>             <C>
Barges, boats and related equipment.........................    $217,173        $183,367
Land and buildings..........................................      12,453           7,908
Machinery and equipment.....................................         245             245
Office furniture and equipment..............................       3,183           1,751
Leasehold improvements......................................       1,983           1,787
                                                                --------        --------
                                                                 235,037         195,058
Less--Accumulated depreciation..............................     (25,536)        (18,163)
                                                                --------        --------
Property and equipment, net.................................    $209,501        $176,895
                                                                ========        ========
</Table>

    During the nine months ended September 30, 2001, we incurred $40.0 million
of capital expenditures primarily related to the purchase and upgrade of the SEA
HORIZON, the purchase and upgrade of the PECOS HORIZON and the continued
construction of the spool base at our Port Arthur facility.

    On January 1, 2000, we changed depreciation methods from the straight-line
method to the units-of-production method on our major barges and vessels to more
accurately reflect the wear and tear of normal use. We believe that the
units-of-production method is best suited to reflect the actual deterioration of
our marine equipment. Depreciation expense calculated under the
units-of-production method may be different than depreciation expense calculated
under the straight-line method in any period. The annual depreciation based on
utilization of each vessel will not be less than 25% of annual straight-line
depreciation, and the cumulative depreciation based on utilization of each
vessel will not be less than 50% of cumulative straight-line depreciation. For
the nine months ended September 30, 2000, we recorded the cumulative effect of a
$1.4 million adjustment, or $0.07 per share, which is net of taxes of
$0.7 million related to changing depreciation methods from the straight-line
method to the units-of-production method.

3.  STOCKHOLDERS' EQUITY

STOCK OFFERING

    In February 2001, we sold 3,800,000 shares of common stock in a secondary
offering. We received $66.0 million after deducting the underwriting discount
and expenses. We used $30.0 million to reduce indebtedness, and $5.6 million for
the initial purchase of the PECOS HORIZON, a 256' dedicated bury barge. This
barge is currently being converted to a pipelay/pipebury barge at our Port
Arthur facility. The remainder is being used for general corporate purposes,
including funding capital expenditures to expand our operating capabilities,
potential vessel acquisitions and vessel upgrades, and to support our joint
venture with Cal Dive International.

EARNINGS PER SHARE

    Earnings per share data for all periods presented has been computed pursuant
to Statement of Financial Accounting Standards (SFAS) No. 128, "Earnings Per
Share" that requires a presentation of basic earnings per share (basic EPS) and
diluted earnings per share (diluted EPS). Basic EPS excludes

                                       6
<Page>
                    HORIZON OFFSHORE, INC. AND SUBSIDIARIES

             NOTES TO CONSOLIDATED FINANCIAL STATEMENTS (CONTINUED)

dilution and is determined by dividing income available to common stockholders
by the weighted average number of common shares outstanding during the period.
Diluted EPS reflects the potential dilution that could occur if securities and
other contracts to issue common stock were exercised or converted into common
stock. As of September 30, 2001, we had outstanding options covering an
aggregate of 2,541,338 shares of common stock, of which 943,804 shares were
exercisable. Options to purchase approximately 1.4 million shares of common
stock at a weighted average price of $16.08 per share were outstanding as of
September 30, 2001 but were not included in the computation of diluted earnings
per share.

TREASURY STOCK

    In July 1998, the board of directors approved the repurchase of up to
$10.0 million of Horizon's outstanding common stock. Shares were purchased from
time to time in 1998 and 1999, subject to market conditions, in the open market
or in privately negotiated transactions. During the nine months ended
September 30, 2001, we contributed 20,324 shares of treasury stock to our 401(k)
Plan. As of September 30, 2001, our treasury stock consisted of 983,076 shares
at a cost of $6.4 million.

4.  LOSS ON EXTINGUISHMENT OF DEBT

    During March 2001, we paid $30.0 million to reduce the outstanding principal
balance under our term loan, which was subject to a penalty of $0.6 million (net
of taxes of $0.3 million), or $0.02 per share-diluted, for the early
extinguishment of debt.

5.  RELATED PARTY TRANSACTIONS

    In August 1998, we entered into a master service agreement with Odyssea
Marine, Inc. (Odyssea), an entity wholly-owned by Elliott Associates, L.P. and
Elliott International, L.P., to charter certain marine vessels on an as-needed
basis from Odyssea. As of September 30, 2001, Horizon had an outstanding balance
payable to Odyssea of $0.4 million. During the nine months ended September 30,
2001, Odyssea billed Horizon $13.6 million and we paid Odyssea $13.5 million for
services rendered under the agreement. Odyssea billed Horizon $7.0 million and
we paid Odyssea $5.8 million during the nine months ended September 30, 2000. In
management's opinion, the transactions were effected on terms similar to those
which could have been obtained from unaffiliated third parties.

6.  NOTES PAYABLE

    At September 30, 2001, we had approximately $111.5 million total outstanding
debt. This represents an approximate net increase of $20.5 million from
December 31, 2000. This increase in debt is due to new borrowing facilities we
obtained to finance the acquisition and upgrade of the SEA HORIZON and provide
working capital for our Mexico projects. Of the $111.5 million of outstanding
debt, $43.2 million represents borrowings on our three revolving credit
facilities and $68.3 million represents borrowing on four term-debt facilities.
At September 30, 2001, we had $10.3 million available under our three revolving
credit facilities. Interest rates vary from Prime less 0.5% to Libor plus 2.95%.
Our current average interest rate is 5.45%, and our term-debt borrowings
currently require a $545,085 monthly principal payment.

7.  INCOME TAXES

    For the three and nine months ended September 31, 2001, we recorded a
federal income tax provision of $0.3 million, at a net effective rate of 10.0%,
and a $4.8 million income tax provision, at a net effective rate of 30.7%,
respectively. The decrease in our effective tax rate is due to the application
of the extraterritorial income exclusion in the third quarter of 2001 for income
primarily earned in Mexico from October 1, 2000 to September 30, 2001.

    The extraterritorial income exclusion act of 2000 allows certain income
derived from outside the United States to be excluded from gross income when
calculating a corporation's U.S. tax provision. The exclusion is generally
effective for transactions entered into after September 30, 2000.

                                       7
<Page>
ITEM 2. MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
  OF OPERATIONS

    THE FOLLOWING DISCUSSION AND ANALYSIS SHOULD BE READ IN CONJUNCTION WITH
HORIZON'S CONSOLIDATED FINANCIAL STATEMENTS AND NOTES THERETO AND THE DISCUSSION
UNDER "MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS
OF OPERATIONS" INCLUDED IN OUR 2000 ANNUAL REPORT ON FORM 10-K. THE FOLLOWING
INFORMATION CONTAINS FORWARD-LOOKING STATEMENTS, WHICH ARE SUBJECT TO RISKS AND
UNCERTAINTIES. SHOULD ONE OR MORE OF THESE RISKS OR UNCERTAINTIES MATERIALIZE,
ACTUAL RESULTS MAY DIFFER FROM THOSE EXPRESSED OR IMPLIED BY THE FORWARD-LOOKING
STATEMENTS.

GENERAL

    We provide marine construction services to the offshore oil and gas
industry. We now operate primarily in the Gulf of Mexico, offshore Mexico, and
have recently expanded our operations to Central and South America, and
Southeast Asia. Our marine fleet consists of thirteen vessels, ten of which are
currently operational. We have also established alliances and joint ventures
with selected offshore service and energy companies in order to broaden the
range of services we offer our customers, participate in the ultra-deepwater
market and to expand our operations internationally. The primary services we
provide include:

    - installing pipelines in the Gulf of Mexico and selected international
      markets;

    - providing pipebury, hook-up and commissioning services; and

    - installing production platforms and other structures and then
      disassembling and salvaging them at the end of their life cycles.

    Our operating results are directly tied to industry demand for our services,
most of which are performed on the outer continental shelf in the Gulf of
Mexico, and, to a lesser degree, seasonal impact. Demand for our services is
primarily a function of the level of oil and gas activity in our market areas.
Due to the time required to drill a well and fabricate a production platform,
demand for our services usually lags exploratory drilling by nine to eighteen
months and sometimes longer. We believe our operating results are significantly
affected by current market conditions, our enhanced fleet capabilities and our
management expertise.

RESULTS OF OPERATIONS

    Revenues increased for the nine months ended September 2001 compared with
the nine months ended September 2000 primarily in response to our operations
offshore Mexico. Our current projects in Mexico are slated for completion by the
end of 2001; however, we are also bidding on additional work in this market
area. Profit margins in the Gulf of Mexico have remained very competitive during
the third quarter due to customer concerns over U.S. natural gas pricing trends
and the general slow down of the U.S. economy.

    We continued our international expansion efforts with the purchase of the
SEA HORIZON, a combination, 360' derrick/lay barge, currently located in
Indonesia. After completing the upgrade and modification expected by year-end,
the vessel will be able to perform turnkey projects in remote locations with a
single mobilization, and will be our largest combination barge. We intend to
operate the vessel in Indonesia, Malaysia, as well as other areas in Southeast
Asia such as Thailand and Vietnam.

    The conversion of the PECOS HORIZON to a combination lay/bury barge is
ongoing at our Port Arthur facility. The PECOS HORIZON will be capable of laying
pipelines up to 20-inches in diameter and operating in water depths up to
500-feet. The vessel will also be capable of burying pipelines up to 60-inches
in diameter to depths of 16-feet below the seafloor. The acquisition of this
barge further expands our fleet of construction vessels and will enhance our
strategic position both domestically and internationally.

                                       8
<Page>
QUARTER ENDED SEPTEMBER 30, 2001 COMPARED TO THE QUARTER ENDED SEPTEMBER 30,
  2000

    CONTRACT REVENUES.  Contract revenues were $61.0 million for the quarter
ended September 30, 2001, compared to $56.3 million for the quarter ended
September 30, 2000. Revenues increased slightly due to our ongoing projects
offshore Mexico. Our two projects in Mexico contributed approximately
$29.2 million in revenues during the third quarter.

    GROSS PROFIT.  Gross profit was $7.7 million (12.6% of contract revenues)
for the quarter ended September 30, 2001 compared to gross profit of
$10.3 million (18.2% of contract revenues) for the quarter ended September 30,
2000. Gross profit as a percentage of revenues decreased due to lower margins
bid on jobs as a result of the competitive market conditions in the Gulf of
Mexico.

    SELLING, GENERAL AND ADMINISTRATIVE EXPENSES.  Selling, general and
administrative expenses increased to $3.0 million (5.0% of contract revenues)
for the three months ended September 30, 2001, compared with $2.4 million (4.3%
of contract revenues) for the same period in 2000. The percentage increase was
primarily due to the addition of personnel and other expenses associated with
the Company's continuing international expansion and the fleet expansion.

    INTEREST EXPENSE.  Interest expense was $1.3 million for the three months
ended September 30, 2001, net of $0.4 million interest capitalized, and
$2.1 million for the same period last year, net of $0.1 million interest
capitalized. Our total outstanding debt was $111.5 million at September 30,
2001, compared to $93.1 million at September 30, 2000.

    INTEREST INCOME.  Interest income on cash investments for the three months
ended September 30, 2001 was $76,000 compared to $80,000 for the third quarter
in 2000.

    OTHER INCOME (EXPENSE).  Other income (expense) for the three months ended
September 30, 2001 primarily consisted of approximately $352,000 of foreign
currency loss and other expense of approximately $33,000 related to our Cal Dive
joint venture.

    INCOME TAXES.  We use the liability method of accounting for income taxes.
For the quarter ended September 30, 2001, we recorded a federal income tax
provision of $0.3 million, at a net effective rate of 10.0% on pre-tax net
income of $3.1 million. For the quarter ended September 30, 2000, we recorded a
federal income tax provision of $2.1 million, at a net effective rate of 36.3%
on pre-tax net income of $5.8 million. The decrease in our effective tax rate is
due to the cumulative effect of the application of the extraterritorial income
exclusion for income earned primarily in Mexico from October 1, 2000 to
September 30, 2001. The exclusion allows us to exclude a portion of income
earned outside the United States for tax purposes.

    NET INCOME.  Net income for the quarter ended September 30, 2001 was
$2.8 million, or $0.12 per share-diluted. This compares with net income of
$3.7 million, or $0.19 per share for the quarter ended September 30, 2000.

NINE MONTHS ENDED SEPTEMBER 30, 2001 COMPARED TO THE NINE MONTHS ENDED
  SEPTEMBER 30, 2000

    CONTRACT REVENUES.  Contract revenues were $198.4 million for the nine
months ended September 30, 2001, compared to $108.8 million for the nine months
ended September 30, 2000. The increase in revenues is primarily due to our
ongoing projects offshore Mexico. Our two projects in Mexico contributed
approximately $105.8 million in revenues for the nine months ended
September 30, 2001.

    GROSS PROFIT.  Gross profit was $30.2 million (15.2% of contract revenues)
for the first nine months of 2001, compared to a gross profit of $17.9 million
(16.5% of contract revenues) for the same period of 2000. Gross profit as a
percentage of revenues decreased due to lower margins bid on jobs as a result of
the competitive market conditions in the Gulf of Mexico.

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    SELLING, GENERAL AND ADMINISTRATIVE EXPENSES.  Selling, general and
administrative expenses were $10.1 million (5.1% of contract revenues) for the
nine months ended September 30, 2001, compared to $6.8 million (6.2% of contract
revenues) for the same period of 2000. The percentage decrease was primarily due
to the increase in revenues with a lower corresponding increase in selling,
general and administrative expenses in the first half of 2001.

    INTEREST EXPENSE.  Interest expense for the nine months ended September 30,
2001 was $4.4 million net of $0.6 million of interest capitalized. Interest
expense was $5.6 million for the nine months ended September 30, 2000, net of
$0.3 million on interest capitalized.

    INTEREST INCOME.  Interest income on cash investments for the nine months
ended September 30, 2001 and 2000 was $369,000 and $232,000, respectively.

    OTHER INCOME (EXPENSE).  Included in other income (expense) for the nine
months ended September 30, 2001 is $190,000 of foreign currency loss, $119,000
related to Mexico operations, $34,000 of expenses related to our Cal Dive joint
venture and $39,000 of equity losses related to the DSND joint venture. The DSND
joint venture was dissolved in February 2001.

    INCOME TAXES.  We use the liability method of accounting for income taxes.
For the nine months ended September 30, 2001, we recorded a federal income tax
provision of $4.8 million, at a net effective rate of 30.7% on a pre-tax income
of $15.7 million. For the same period last year, we recorded a federal income
tax provision of $2.1 million, at a net effective rate of 36.8% on a pre-tax
income of $5.7 million. The decrease in our effective tax rate is due to the
cumulative effect of the application of the extraterritorial income exclusion in
the third quarter of 2001 for income earned primarily in Mexico from October 1,
2000 to September 30, 2001. The exclusion allows us to exclude a portion of
income earned outside the United States for tax purposes.

    EXTRAORDINARY LOSS.  For the nine months ended September 30, 2001, we
recorded an extraordinary loss of $0.6 million net of taxes of $0.3 million, or
$0.02 per share-diluted, related to a penalty on the early extinguishment of
debt. See Note 4 of the notes to consolidated financial statements.

    CUMULATIVE EFFECT OF ACCOUNTING CHANGE.  We recorded a cumulative effect of
$1.4 million net of taxes of $0.7 million, or $0.07 per share, related to
changing depreciation methods from straight-line to the units-of-production
method effective January 1, 2000. See Note 2 of the notes to consolidated
financial statements.

    NET INCOME.  Net income was $10.3 million, or $0.45 per share-diluted, for
the nine months ended September 30, 2001, which includes the effect of an
extraordinary loss of $0.6 million, or $0.02 per share-diluted, related to the
early extinguishment of debt. Net income for the nine months ended
September 30, 2000 was $5.0 million, or $0.26 per share, which includes the
effect of a cumulative adjustment of $1.4 million, or $0.07 per share, related
to a change in accounting principle.

LIQUIDITY AND CAPITAL RESOURCES

    In February 2001, we sold 3,800,000 shares of common stock in a secondary
offering. We received $66.0 million after deducting the underwriting discount
and expenses. We used $30.0 million to reduce indebtedness, and $5.6 million for
the initial purchase of the PECOS HORIZON, a 256' dedicated bury barge. This
barge is currently being converted to a pipelay/pipebury barge at our Port
Arthur facility. The remainder is being used for general corporate purposes,
including funding capital expenditures to expand our operating capabilities,
potential vessel acquisitions and vessel upgrades, and to support our joint
venture with Cal Dive International.

    Our primary liquidity needs are to fund acquisitions and improvements to the
fleet necessary to expand operations and to provide working capital. We had
$85.6 million of working capital at

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September 30, 2001, compared to $10.0 million of working capital at
December 31, 2000. The increase in working capital was attributable to increased
costs in excess of billings from our Mexico operations and a decrease in current
maturities of long-term debt, which resulted primarily from the early
extinguishment of debt. The asset "Cost in Excess of Billings" represents
revenues recognized in excess of amounts billed. Our Mexico contracts require us
to reach certain milestones before issuing progress billings to the customer. We
believe we will complete a majority of these milestones by year-end, which will
allow us to issue progress billings and receive payment. Cash used in operations
was $50.1 million for the nine months ended September 30, 2001, compared to
$3.8 million of cash provided by operations for the nine months ended
September 30, 2000. Cash used in operations for the quarter ended September 30,
2001 was primarily related to the working capital requirements to support our
projects in Mexico.

    At September 30, 2001, we had approximately $111.5 million total outstanding
debt. This represents an approximate net increase of $20.5 million from
December 31, 2000. This increase in debt is due to new borrowing facilities we
obtained to finance the acquisition and upgrade of the SEA HORIZON and provide
working capital for our Mexico projects. Of the $111.5 million of outstanding
debt, $43.2 million represents borrowings on our three revolving credit
facilities and $68.3 million represents borrowing on four term-debt facilities.
At September 30, 2001, we had $10.3 million available under our three revolving
credit facilities. Interest rates vary from Prime less 0.5% to Libor plus 2.95%.
Our current average interest rate is 5.45%, and our term-debt borrowings
currently require a $545,085 monthly principal payment.

    Our loans require that certain conditions be met in order for us to obtain
advances. Our loans are secured by mortgages on certain vessels and by accounts
receivable. Advances under our revolving credit facilities may be obtained in
accordance with a borrowing base defined as a percentage of accounts receivable
balances. The loans contain customary defaults and some require us to maintain
certain financial ratios. The loan agreements also contain certain covenants
that limit our ability to incur additional debt, pay dividends, create certain
liens, sell assets and make capital expenditures. We were in compliance with all
loan covenants as of September 30, 2001.

    Current maturities of long-term debt were $7.2 million as of September 30,
2001. Our long-term debt will require approximately $7.2 million in principal
payments for 2002. We believe that cash generated from operations and the
offering, together with available borrowings under our revolving credit
facilities, will be sufficient to fund the planned capital projects and working
capital requirements into 2002. Planned capital expenditures for the remainder
of 2001 are estimated to total approximately $13.5 million. Our strategy,
however, is to make other acquisitions, to expand our operating capabilities and
expand into additional selected international areas. To the extent we are
successful in identifying acquisition and expansion opportunities and have
expanded our market share in those areas, we may require additional equity or
debt financing depending on the size of any transaction.

FORWARD-LOOKING STATEMENTS

    In addition to historical information, Management's Discussion and Analysis
of Financial Condition and Results of Operations includes certain
forward-looking statements regarding events and financial trends that may affect
our future operating results and financial position. Some important factors that
could cause actual results to differ materially from the anticipated results or
other expectations expressed in our forward-looking statements include the
following:

    - industry volatility, including the level of capital expenditures by oil
      and gas companies due to fluctuations in the price of oil and gas;

    - risks of growth strategy, including the risks of rapid growth;

    - operating hazards, including the unpredictable effect of natural
      occurrences on operations and the significant possibility of accidents
      resulting in personal injury and property damage;

    - the highly competitive nature of the marine construction business;

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    - dependence on the continued strong working relationships with significant
      customers operating in the Gulf;

    - seasonality of the offshore construction industry in the Gulf;

    - the need for additional financing;

    - contract bidding risks;

    - percentage-of-completion accounting;

    - continued active participation of our executive officers and key operating
      personnel;

    - the effect on our performance of regulatory programs and environmental
      matters;

    - risks involved in the expansion of our operations into international
      offshore oil and gas producing areas; and

    - risks involved in joint venture operations, including difficulty in
      resolving disputes with present partners or reaching agreements with
      future partners.

    Many of these factors are beyond our ability to control or predict. We
caution investors not to place undue reliance on forward-looking statements. We
disclaim any intent or obligation to update the forward-looking statements
contained in this report, whether as a result of receiving new information, the
occurrence of future events or otherwise.

    These and other uncertainties related to the business are described in
detail under the heading "Cautionary Statement" in our 2000 Annual Report on
Form 10-K.

ITEM 3. QUANTITATIVE AND QUALITATIVE DISCLOSURES ABOUT MARKET RISK

    Our market risk exposures primarily include interest rate and exchange rate
fluctuation on derivative and financial instruments as detailed below. Our
market risk sensitive instruments are classified as "other than trading". The
following sections address the significant market risks associated with our
financial activities during the nine months ended September 30, 2001. Our
exposure to market risk as discussed below includes "forward-looking statements"
and represents estimates of possible changes in fair values, future earnings or
cash flows that would occur assuming hypothetical future movements in foreign
currency exchange rates or interest rates.

    As of September 30, 2001 the carrying value of our long-term variable rate
debt, including accrued interest, was approximately $111.5 million. The fair
value of this debt approximates the carrying value because the interest rates
are based on floating rates identified by reference to market rates. Fair value
was determined as noted above. A hypothetical 1% increase in the applicable
interest rates as of September 30, 2001 would increase annual interest expense
by approximately $1.1 million.

    In Mexico, we collect revenues and pay expenses in foreign currency. We
monitor the exchange rate of our foreign currencies and, when deemed
appropriate, enter into hedging transactions in order to mitigate the risk from
foreign currency fluctuations. We also manage foreign currency risk by
attempting to contract as much foreign revenue as possible in U.S. Dollars.

    Our revenues generally do not depend on any one customer. The level of
construction services required by a customer depends on the size of their
capital expenditure budget for construction for the year. Consequently,
customers that account for a significant portion of contract revenues in one
year may represent an immaterial portion of contract revenues in subsequent
years.

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                           PART II--OTHER INFORMATION

ITEM 6. EXHIBITS AND REPORTS ON FORM 8-K.

    (a) Exhibits

     3.1   Amended and Restated Certificate of Incorporation of the Company(1)

     3.2   Bylaws of the Company(1)

    10.1   Loan Agreement dated August 15, 2001, between Horizon Offshore
           Contractors, Inc. Horizon Subsea Services, Inc., Horizon
           Vessels, Inc., Horizen, L.L.C. and Southwest Bank of Texas, N.A.(2)

    10.2   Loan Agreement dated August 31, 2001, between Horizon Vessels
           International Ltd. and GMAC Business Credit, LLC(2)

    (b) Reports on Form 8-K:

    On July 23, 2001, we filed a report on Form 8-K, reporting under item 5,
announcing earnings for the second quarter ended June 30, 2001.

    On July 23, 2001, we filed a report on Form 8-K, reporting under item 5,
announcing the purchase of Derrick/Lay Barge in Southeast Asia.

------------------------

(1) Incorporated by reference to our Registration Statement on Form S-1
    (Registration Statement No. 333-43965).

(2) Filed herewith

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                                   SIGNATURE

    Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned thereunto duly authorized.

<Table>
<S>                                                    <C>  <C>
                                                       HORIZON OFFSHORE, INC.

Date: November 14, 2001                                By:              /s/ DAVID W. SHARP
                                                            -----------------------------------------
                                                                          David W. Sharp
                                                                   EXECUTIVE VICE PRESIDENT AND
                                                                     CHIEF FINANCIAL OFFICER
</Table>

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