<PAGE>

                                                                  EXHIBIT 10.33
                                                                  -------------

                      FIRST AMENDMENT TO CREDIT AGREEMENT
                 AND AMENDMENT TO REVOLVING NOTE AND TERM NOTE
                 ---------------------------------------------


     This First Amendment to Credit Agreement and Amendment to Revolving Note
and Term Note is made as of this 23rd day of March, 2001, by and between Cape
Cod Bank and Trust Company ("Lender"), a Massachusetts banking corporation, with
a principal place of business at 2 Barlows Landing Road, Pocasset, Massachusetts
02559 and Benthos, Inc. ("Borrower"), a Massachusetts corporation with its
principal place of business at 49 Edgerton Drive, North Falmouth, Massachusetts
02556.

                                R E C I T A L S:
                                - - - - - - - -

     A.  Borrower and Lender entered into a certain Credit Agreement dated
August 18, 1999 regarding certain Revolving Loans and a Term Loan as defined
therein, pursuant to which Borrower executed and delivered to Lender a Revolving
Note and a Term Note;

     B.  Borrower and Lender have previously amended the Revolving Note pursuant
to an Amendment of Promissory Note dated December 8, 2000, and amended the Term
Note pursuant to an Amendment of Commercial Variable Rate Promissory Note dated
October 17, 2000.

     C.  Borrower and Lender now desire to amend the Credit Agreement and to
further amend each of the Revolving Note and the Term Note as set forth herein
and simultaneously herewith Borrower and Lender shall execute and deliver a
Second Amendment To Commercial Variable Rate Revolving or Draw Note in the form
of Exhibit A attached hereto and a Second Amendment to Commercial Variable Rate
Promissory Note in the form of Exhibit B attached hereto.

                              A G R E E M E N T S:
                              - - - - - - - - - -

     Now, therefore, in consideration of the foregoing and for other good and
valuable consideration, the receipt and sufficiency of which is hereby
acknowledged, the Lender and the Borrower hereby agree as follows:

     1.  Capitalized terms used herein shall have the meaning given to them in
the Credit Agreement unless separately defined herein.

     2.  As of March 22, 2001, the outstanding principal balance of this
Revolving Loan under the Revolving Note is Three Hundred Thousand Dollars
($300,000.00) and the outstanding principal balance of the Term Loan under the
Term Note is Four Million Two Hundred Fifty-Five Thousand Nine Hundred Fifty-Two
and 39/100 Dollars ($4,255,952.39).

     3.  The Expiration Date of the Revolving Loan (and the maturity date of the
Revolving Note) is hereby extended to January 31, 2002.
<PAGE>

     4.  The Revolving Commitment Amount is hereby reduced from Two Million
Dollars ($2,000,000.00) to Seven Hundred Thousand Dollars ($700,000.00).

     5.  The rate of interest payable on the Revolving Loans is hereby changed
to a variable rate per annum equal to the Prime Rate plus one and one-half
percent (1  1/2%).  Accordingly, the second sentence of Section 2.04(a) of the
Credit Agreement is hereby deleted and replaced with the following:  "The rate
of interest so payable shall be a fluctuating rate per annum which at all times
shall be equal to the sum of the Prime Rate plus one and one-half percent (1
1/2%), but, in no event, in excess of the maximum rate then permitted by
applicable law, with a change in such rate to become effective on the same day
on which any change in the Prime Rate is effective."

     6.  The rate of interest payable on the Term Loan is hereby changed to a
variable rate per annum equal to the Prime Rate plus one-half percent ( 1/2%).

     7.  Section 8.02 of the Credit Agreement is hereby deleted in its entirety
and replaced with the following:

     "Section 8.02.  Financial Covenants.
                     -------------------

               The Borrower covenants and agrees that, so long as any loan is
          outstanding or any obligation of the Borrower to the Lender, in any
          capacity, remains unpaid, or any commitment by the Lender to the
          Borrower is in effect:

          a.   The Borrower will maintain a ratio of Current Assets to Current
               Liabilities of greater than or equal to 1.50 to 1.00;

          b.   The Borrower shall maintain a ratio of Total Debt to Tangible Net
               Worth less than or equal to 2.00 to 1.00; and

          c.   The Borrower shall maintain a ratio of Cash Flow to Debt Service
               Payments, which on a cumulative basis for the applicable calendar
               year, shall be greater than or equal to the following:

                    (i)    -.50 to 1.00 as of  March 31, 2001;

                    (ii)   .34  to 1.00 as of  June 30, 2001; and

                    (iii)  .74 to 1.00 as of September 30, 2001 and thereafter.

          d.   Borrower shall monthly, on or before the fifteenth (15th) of each
               month for the immediately prior month, provide to Lender an
               accounts receivable aging report, an inventory report and a
               backlog backlog report.

                                       2
<PAGE>

     8.  Except as provided herein, the Credit Agreement shall remain unchanged
and as amended hereby, the Credit Agreement is hereby ratified and confirmed.

Witness:            BENTHOS, INC.


/s/ Francis E. Dunne, Jr.        By: /s/ Stephen D. Fantone
                                     -------------------------
                                     Name:  Stephen D. Fantone
                                     Title: President & CEO

Witness:  CAPE COD BANK AND TRUST  COMPANY



/s/ Stephen Sooy                 By: /s/ Timothy F. Kelleher
-----------------                    ----------------------------
                                     Name:  Timothy F. Kelleher
                                     Title: Senior Vice President

                                       3