<PAGE>

                                                                    EXHIBIT 10.3

                               WAIVER AND CONSENT

      This Waiver and Consent ("Agreement") dated as of December 29, 2004
("Effective Date") is among Mariner Energy, Inc., a Delaware corporation (the
"Borrower"), Mariner Holdings, Inc. and Mariner Energy, LLC, the Lenders (as
defined below), and Union Bank of California, N.A., as administrative agent for
such Lenders (in such capacity, the "Administrative Agent") and as issuing
lender for such Lenders (in such capacity, the "Issuing Lender").

                                    RECITALS

      A.    The Borrower, the Lenders, Issuing Lender and the Administrative
Agent are parties to the Credit Agreement dated as of March 2, 2004, as amended
by that certain Amendment No. 1 and Assignment Agreement dated as of July 14,
2004 (as so amended, the "Credit Agreement").

      B.    On May 10, 2004 the Borrower acquired a 51% interest in an
approximately 29 mile flowline from MEGS, LLC an affiliate of Enron Corp. (such
acquisition, the "MEGS Acquisition") as more particularly described in that
certain Purchase and Sale Agreement dated as of April 28, 2004 by and among
MEGS, LLC, W & T Offshore, Inc. and the Borrower ("MEGS Acquisition Agreement").

      C.    On August 5, 2004 the Borrower acquired six offshore blocks in the
Gulf of Mexico from Juniper Energy L.P., an affiliate of Enron Corp. (such
acquisition, the "Juniper Acquisition") as more particularly described in that
certain Purchase and Sale Agreement dated as of August 5, 2004 by and between
Juniper Energy L.P. and the Borrower ("Juniper Acquisition Agreement").

      D.    At the request of the Borrower, the Administrative Agent and the
Lenders wish to, subject to the terms and conditions of this Agreement, provide
for a waiver of the Defaults and Events of Default (each as defined in the
Credit Agreement) existing as a result of the MEGS Acquisition and the Juniper
Acquisition (collectively, the "Acquisitions").

      E.    Furthermore, the Borrower wishes to create two new subsidiaries and
transfer certain assets described in Schedule A attached hereto (the "Texas
Assets") to one of such subsidiaries in the manner described in said Schedule A.

      F.    At the request of the Borrower, the Administrative Agent and the
Lenders wish to, subject to the terms and conditions of this Agreement, consent
to (i) the creation of such new subsidiaries, (ii) such transfer of the Texas
Assets and (iii) the amendment and restatement of the Borrower's bylaws in the
form attached as Exhibit A to this Agreement (the "Amended and Restated
Bylaws").

      THEREFORE, the Borrower, the Lenders, the Issuing Lender and the
Administrative Agent hereby agree as follows:

<PAGE>

                                   ARTICLE I.
                                   DEFINITIONS

      SECTION 1.01 TERMS DEFINED ABOVE. As used in this Agreement, each of the
terms defined in the opening paragraph and the Recitals above shall have the
meanings assigned to such terms therein.

      SECTION 1.02 TERMS DEFINED IN THE CREDIT AGREEMENT. Each term defined in
the Credit Agreement and used herein without definition shall have the meaning
assigned to such term in the Credit Agreement, unless expressly provided to the
contrary.

      SECTION 1.03 OTHER DEFINITIONAL PROVISIONS. The words "hereby", "herein",
"hereinafter", "hereof", "hereto" and "hereunder" when used in this Agreement
shall refer to this Agreement as a whole and not to any particular Article,
Section, subsection or provision of this Agreement. Article, Section, subsection
and Exhibit references herein are to such Articles, Sections, subsections and
Exhibits of this Agreement unless otherwise specified. All titles or headings to
Articles, Sections, subsections or other divisions of this Agreement or the
exhibits hereto, if any, are only for the convenience of the parties and shall
not be construed to have any effect or meaning with respect to the other content
of such Articles, Sections, subsections, other divisions or exhibits, such other
content being controlling as the agreement among the parties hereto. Whenever
the context requires, reference herein made to the single number shall be
understood to include the plural; and likewise, the plural shall be understood
to include the singular. Words denoting sex shall be construed to include the
masculine, feminine and neuter, when such construction is appropriate; and
specific enumeration shall not exclude the general but shall be construed as
cumulative. Definitions of terms defined in the singular or plural shall be
equally applicable to the plural or singular, as the case may be, unless
otherwise indicated.

                                   ARTICLE II.
                               WAIVER AND CONSENT

      SECTION 2.01 WAIVER. The Borrower hereby acknowledges the existence of the
following Defaults and Events of Default (collectively, the "Waiver Defaults"):
(a) the Borrower's violation of Section 6.22 of the Credit Agreement as a result
of the entering into the MEGS Acquisition Agreement and the consummation of the
MEGS Acquisition without the prior written consent of the Required Lenders, and
(b) the Borrower's violation of Section 6.22 of the Credit Agreement as a result
of the entering into the Juniper Acquisition Agreement and the consummation of
the Juniper Acquisition without the prior written consent of the Required
Lenders. Subject to the terms of this Agreement, the Lenders hereby waive the
Waiver Defaults.

      SECTION 2.02 CONSENTS. Subject to the terms of this Agreement, the Lenders
also hereby consent to (a) the Borrower creating a new wholly-owned subsidiary
that will be a Delaware limited liability company ("Delaware LLC"), (b) the
Borrower and such newly formed Delaware LLC creating a new subsidiary that will
be Delaware limited partnership ("Delaware LP") and of which the Delaware LLC
will hold the 99% limited partner interest and the Borrower will hold the 1%
general partner interest, (c) the transfer of the Texas Assets as

                                       2-
<PAGE>

detailed in Schedule A as contribution from the Borrower to the Delaware LP, (d)
the initial contribution of cash equity from the Borrower to the Delaware LLC,
and (e) the initial contribution of cash equity from the Delaware LLC to the
Delaware LP. The Administrative Agent hereby consents to the amendment to the
Borrower's bylaws as set forth in the Amended and Restated Bylaws. Each of the
Lenders hereby acknowledges and agrees to such consent by the Administrative
Agent. The consents by the Lenders and by the Administrative described in this
Section 2.02 are collectively referred to herein as the "Consents."

      SECTION 2.03 ACKNOWLEDGMENT AND AGREEMENT. The waiver and consents by the
Lenders described in the preceding Section 2.01 and 2.02 are contingent upon the
satisfaction of the conditions precedent set forth below and are strictly
limited to the Waiver Defaults and the Consents to the extent described herein.
Nothing contained herein shall be construed to be a consent to or a permanent
waiver of the Sections covered by the Waiver Defaults or the Consents or any
other terms, provisions, covenants, warranties or agreements contained in the
Credit Agreement or in any of the other Loan Documents. The Lenders reserve the
right to exercise any rights and remedies available to them in connection with
any other present or future defaults with respect to the Credit Agreement or any
other provision of any Loan Document. The descriptions herein of the Waiver
Defaults and the Consents are based upon the information provided to the Lenders
on or prior to the date hereof and shall not be deemed to exclude the existence
of any other Defaults or Events of Default. The failure of the Lenders to give
notice to the Borrower or the Guarantors of any such other Defaults or Events of
Default is not intended to be nor shall be a waiver thereof. The Borrower hereby
agrees and acknowledges that the Lenders require and will require strict
performance by the Borrower of all of its obligations, agreements and covenants
contained in the Credit Agreement and the other Loan Documents, and no inaction
or action regarding any Default or Event of Default is intended to be or shall
be a waiver thereof.

                                  ARTICLE III.
                         REPRESENTATIONS AND WARRANTIES

      The Borrower represents and warrants that: (a) the representations and
warranties contained in Article IV of the Credit Agreement and the
representations and warranties contained in the Security Instruments, the
Guaranties, and each of the other Loan Documents are true and correct in all
material respects on and as of the Effective Date, before and after giving
effect to the Acquisitions and after giving effect to the terms of this
Agreement, as though made on and as of such date, except those representations
and warranties that speak of a certain date, which representations and
warranties were true and correct as of such date; (b) after giving effect to the
terms of this Agreement, no Default has occurred and is continuing; (c) the
execution, delivery and performance of this Agreement are within the corporate
power and authority of the Borrower and have been duly authorized by appropriate
corporate action and proceedings; (d) this Agreement constitutes a legal, valid,
and binding obligation of the Borrower enforceable in accordance with its terms,
except as limited by applicable bankruptcy, insolvency, reorganization,
moratorium, or similar laws affecting the rights of creditors generally and
general principles of equity; (e) there are no governmental or other third party
consents, licenses and approvals required in connection with the execution,
delivery, performance, validity and enforceability of this Agreement; (f) the
Liens under the Security Instruments are valid and subsisting and secure
Borrower's obligations under the Loan Documents; and (g) the Amended and
Restated Bylaws attached as Exhibit A hereto is the true, correct and complete
copy of the

                                       3-
<PAGE>

bylaws of the Borrower, and the same has not been amended, supplemented, or
otherwise modified and remains in full force and effect as of the date hereof.

                                   ARTICLE IV.
                                   CONDITIONS

      This Agreement and the waiver and consents contained herein shall become
effective and enforceable against the parties hereto upon the occurrence of the
following conditions precedent on or before the Effective Date:

      SECTION 4.01 DOCUMENTS; CERTIFICATES. The Administrative Agent shall have
received multiple original counterparts, as requested by the Administrative
Agent, of:

            (a) this Agreement duly and validly executed and delivered by duly
authorized officers of the Borrower, the Administrative Agent, and the Required
Lenders,

            (b) amendments to existing Mortgages or new Mortgages and Security
Agreements, in form and substance satisfactory to the Administrative Agent,
covering the assets acquired under the Acquisitions,

            (c) a supplement to the Pledge Agreement dated as of March 2, 2004
made by the Borrower and the Delaware LLC in form and substance satisfactory to
the Administrative Agent and covering the equity interest to be held by the
Borrower in the Delaware LLC and the Delaware LP and the equity interest to be
held by the Delaware LLC in the Delaware LP,

            (d) a supplement to the Security Agreement dated as of March 2, 2004
made by the Delaware LLC and the Delaware LP in form and substance satisfactory
to the Administrative Agent and covering certain personal property assets of the
Delaware LLC and Delaware LP,

            (e) a new Mortgage executed by the Delaware LP in form and substance
satisfactory to the Administrative Agent and covering the Texas Assets that are
to be contributed by the Borrower to the Delaware LP,

            (f) new Guaranties executed by the Delaware LLC and Delaware LP in
form and substance satisfactory to the Administrative Agent,

            (g) a certificate of the secretary of the Borrower certifying:

            (i) the names and true signatures of officers of the Borrower and
      the Delaware LLC authorized to sign this Agreement, if applicable, and the
      other Loan Documents to be executed by the Borrower, the Delaware LP or
      the Delaware LLC in connection herewith,

            (ii) true, correct and complete copies of (A) the resolutions of the
      board of directors of the Borrower, on behalf of the Borrower for itself
      and as the sole member of the Delaware LLC and as the sole general partner
      of the Delaware LP, authorizing and approving the execution, delivery and
      performance of this Agreement and the other Loan

                                       4-
<PAGE>

      Documents delivered in connection herewith, (B) the certificate of
      formation of Delaware LLC, (C) the limited liability agreement and all
      other governing documents of Delaware LLC, (D) the certificate of limited
      partnership of Delaware LP, (E) the limited partnership agreement and
      other governing documents of Delaware LP, (F) all other documents
      evidencing other necessary corporate action and governmental approvals, if
      any, with respect to the Loan Documents delivered in connection herewith,
      (G) certificates of good standing and existence for Delaware LLC and
      Delaware LP from the State of Delaware and, with respect to the Delaware
      LP only, the State of Texas, and any other state in which foreign
      qualification thereof is necessary or desirable, and (H) purchase and sale
      agreements, bills of sale, assignments and all other documents,
      instruments, and agreements evidencing or effecting the transfer of the
      Texas Assets from the Borrower to the Delaware LP (the "Texas Transfer
      Documents"), and

            (iii) that there are no material contracts, agreements, instruments,
      or documents binding on the Delaware LLC or Delaware LP or any of their
      respective properties other than the Loan Documents, the Texas Transfer
      Documents, and such other contracts, agreements, instruments, and
      documents that are listed in the Security Instruments affecting the Texas
      Assets and entered into prior to the date hereof or copies of which have
      been delivered to the Administrative Agent prior to the date hereof, and

            (h) such other instruments, documents and amendments or supplements
to existing Security Instruments as the Administrative Agent may reasonably
request.

      SECTION 4.02 NO DEFAULT. Other than the Waiver Defaults, no Default shall
have occurred and be continuing as of the Effective Date.

      SECTION 4.03 REPRESENTATIONS. The representations and warranties in this
Agreement shall be true and correct in all material respects.

      SECTION 4.04 FEES AND EXPENSES. The Borrower shall have paid to the
Administrative Agent the fees and expenses payable to it pursuant to the Credit
Agreement, including without limitation, all amounts previously invoiced and all
amounts owing in respect of the preparation, and negotiation of this Agreement
and the other related Loan Documents.

                                   ARTICLE V.
                                  MISCELLANEOUS

      SECTION 5.01 EFFECT ON LOAN DOCUMENTS; ACKNOWLEDGMENTS.

            (a) The Borrower acknowledges that on the date hereof all
Obligations are payable without defense, offset, counterclaim or recoupment.

            (b) The Administrative Agent, the Issuing Lender, and the Lenders
hereby expressly reserve all of their rights, remedies, and claims under the
Loan Documents. Nothing in this Agreement shall constitute a waiver or
relinquishment of (i) any Default or Event of Default under any of the Loan
Documents other than as expressly set forth above, (ii) any of the agreements,
terms or conditions contained in any of the Loan Documents other than as
expressly set forth above, (iii) any rights or remedies of the Administrative
Agent, the Issuing Lender or

                                       5-
<PAGE>

any Lender with respect to the Loan Documents, or (iv) the rights of the
Administrative Agent, any Issuing Lender or any Lender to collect the full
amounts owing to them under the Loan Documents.

            (c) Each of the Borrower, Administrative Agent, Issuing Lender, and
Lenders does hereby adopt, ratify, and confirm the Credit Agreement, and
acknowledges and agrees that the Credit Agreement and all other Loan Documents
are and remain in full force and effect, and the Borrower acknowledges and
agrees that its liabilities under the Credit Agreement and the other Loan
Documents are not impaired in any respect by this Agreement or the waiver and
consents granted hereunder.

            (d) This Agreement is a Loan Document for the purposes of the
provisions of the other Loan Documents. Without limiting the foregoing, any
breach of representations, warranties, and covenants under this Agreement shall
be a Default or Event of Default, as applicable, under the Credit Agreement.

      SECTION 5.02 COUNTERPARTS. This Agreement may be signed in any number of
counterparts, each of which shall be an original and all of which, taken
together, constitute a single instrument. This Agreement may be executed by
facsimile signature and all such signatures shall be effective as originals.

      SECTION 5.03 SUCCESSORS AND ASSIGNS. This Agreement shall be binding upon
and inure to the benefit of the Lenders, the Borrower and the Administrative
Agent hereto and their respective successors and assigns permitted pursuant to
the Credit Agreement.

      SECTION 5.04 INVALIDITY. In the event that any one or more of the
provisions contained in this Agreement shall for any reason be held invalid,
illegal or unenforceable in any respect, such invalidity, illegality or
unenforceability shall not affect any other provision of this Agreement.

      SECTION 5.05 GOVERNING LAW. This Agreement shall be deemed to be a
contract made under and shall be governed by and construed in accordance with
the laws of the State of Texas.

      SECTION 5.06 ENTIRE AGREEMENT. THIS AGREEMENT, THE CREDIT AGREEMENT AS
AMENDED BY THIS AGREEMENT, THE NOTES, AND THE OTHER LOAN DOCUMENTS CONSTITUTE
THE ENTIRE UNDERSTANDING AMONG THE PARTIES HERETO WITH RESPECT TO THE SUBJECT
MATTER HEREOF AND SUPERSEDE ANY PRIOR AGREEMENTS, WRITTEN OR ORAL, WITH RESPECT
THERETO.

            THERE ARE NO UNWRITTEN ORAL AGREEMENTS AMONG THE PARTIES.

                         [SIGNATURES BEGIN ON NEXT PAGE]

                                       6-
<PAGE>

      EXECUTED effective as of the date first above written.

                                           MARINER ENERGY, INC.

                                  By: /s/ Rick G. Lester
                                     ------------------------------------------
                                     Rick G. Lester
                                     Vice President and Chief Financial Officer

                                  MARINER ENERGY LLC

                                  By: /s/ Rick G. Lester
                                     ------------------------------------------
                                      Rick G. Lester
                                      Vice President and Chief Financial Officer

                                  MARINER HOLDINGS, INC.

                                  By: /s/ Rick G. Lester
                                     ------------------------------------------
                                     Rick G. Lester
                                     Vice President and Chief Financial Officer

                                  UNION BANK OF CALIFORNIA, N.A.,
                                  as Administrative Agent, as Issuing Lender,
                                  and as a Lender

                                  By: /s/ Damien Meiburger
                                     ------------------------------------------
                                     Damien Meiburger, Senior Vice President

                                  By: /s/ Ali Ahmed
                                     ------------------------------------------
                                  Name:   Ali Ahmed
                                  Title:  Vice President

                      Signature page to Waiver and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                  BNP PARIBAS

                                  By: /s/ Polly Schott
                                     ------------------------------------------
                                  Name:   Polly Schott
                                  Title:  Vice President

                                  By: /s/ Gabe Ellisor
                                     ------------------------------------------
                                  Name:   Gabe Ellisor
                                  Title:  Vice President

                      Signature page to Waiver and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                  JPMorgan Chase Bank, N.A. (successor by merger
                                  to Bank One, NA)

                                  By: /s/ Jo Linda Papadakis
                                     ------------------------------------------
                                  Name:   Jo Linda Papadakis
                                  Title:  Associate Director

                      Signature page to Waiver and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                  SOUTHWEST BANK OF TEXAS, N.A.

                                  By: /s/ Kenneth R. Batson, III
                                     ------------------------------------------
                                  Name:   Kenneth R. Batson, III
                                  Title:  Vice President, Energy Lending

                      Signature page to Waiver and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                          GUARANTY BANK

                                          By: /s/ Jonathan Gregory
                                              ----------------------------------
                                          Name:   Jonathan Gregory
                                          Title:  Senior Vice President

                      Signature page to Waiver and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                      HARRIS NESBITT FINANCING, INC.

                                      By: /s/ James V. Ducote
                                          ----------------------------------
                                      Name:   James V. Ducote
                                      Title:  Vice President

                      Signature page to Waiver and Consent
                             (Mariner Energy, Inc.)

<PAGE>

                                   SCHEDULE A

            to Waiver and Consent dated as of December 29, 2004 by and among
            Mariner Energy, Inc., Mariner Holdings, Inc. and Mariner Energy,
            LLC, the Lenders (as defined therein), and Union Bank of California,
            N.A., as administrative agent for such Lenders and as issuing lender
            for such Lenders

Mariner Energy, Inc. ("MEI") is transferring to Mariner Energy Texas L.P.
("Mariner Texas"), a Delaware limited partnership, all beneficial interests in
oil and gas leases and other oil and gas assets located in the State of Texas
now owned or hereafter acquired by MEI (the "Texas Assets"). MEI will be the
sole general partner of Mariner Texas and will own a 1% general partnership
interest in Mariner Texas. Mariner LP LLC ("Mariner LP"), a Delaware limited
liability company and wholly owned subsidiary of MEI, will be the sole limited
partner of Mariner Texas and will own a 99% limited partnership interest in
Mariner Texas. A diagram of the proposed ownership structure is attached hereto
as Attachment "1".

As owner of all beneficial interests in the Texas Assets, Mariner Texas will
have the right to receive all revenues and proceeds and the obligation to pay
all expenses and costs from the Texas Assets. If requested by Mariner Texas and
after compliance with any limitations on assignment contained in any contract,
agreement, governmental authorization or lease, MEI will convey title to the
Texas Assets to Mariner Texas. The consent to which this Schedule is attached
covers both the transfer of beneficial interests in the Texas Assets to Mariner
Texas and, if requested by Mariner Texas, the conveyance of title to the Texas
Assets to Mariner Texas.

                                   Schedule A