<Page>
[DECHERT LETTERHEAD]
August 30, 2001
Great Lakes Dredge & Dock Corporation
2122 York Road
Oak Brook, Illinois 60523
Re: Registration Statement on Form S-4
SEC File No. 333-60300
Gentlemen and Ladies:
We have acted as special counsel to Great Lakes Dredge & Dock
Corporation, a Delaware corporation (the "Company") and Great Lakes Dredge &
Dock Company, a New Jersey corporation; Great Lakes Caribbean Dredging, Inc., a
Delaware corporation; Dawson Marine Services Company, a Delaware corporation;
North American Site Developers, Inc., a Massachusetts corporation, and
Fifty-Three Dredging Corporation, a New Jersey corporation (collectively, the
"Guarantors") in connection with the preparation and filing of the Registration
Statement on Form S-4 (Registration No. 333-60300), originally filed on May 4,
2001 with the Securities and Exchange Commission under the Securities Act of
1933, as amended (the "Securities Act"), and the Trust Indenture Act of 1939,
as amended, as subsequently amended by an amendment to be filed today (the
"Registration Statement"). The Registration Statement relates to the proposed
issuance of up to $155 million aggregate principal amount of Series D 11 1/4%
Senior Subordinated Notes due 2008 (the "Exchange Notes") of the Company and
the guarantee thereof (the "Exchange Guarantees") by each of the Guarantors.
The Exchange Notes are to be issued in exchange for an equal aggregate
principal amount of the Company's outstanding Registered Series B 11 1/4%
Senior Subordinated Notes due 2008 and Unregistered Series C 11 1/4% Senior
Subordinated Notes due 2008 (together, the "Existing Notes") and the
Guarantors' guarantees thereof pursuant to the Registration Rights Agreement
dated April 24, 2001 among the Company, the Guarantors and Credit Suisse First
Boston Corporation. The Exchange Notes are to be issued pursuant to the terms
of an Indenture, filed as Exhibit 4.01 to the Registration Statement as
supplemented by the Supplemental Indenture, filed as Exhibit 4.05 to the
Registration Statement (the "Indenture"), among the Company, the Guarantors and
The Bank of New York, as trustee.
We have participated in the preparation of the Registration Statement
and have made such legal and factual examination and inquiry which we have
deemed advisable for the rendering of the opinions set forth below. In making
our examination, we have assumed the
<Page>
Great Lakes Dredge & Dock Corporation
August __, 2001
Page 2
genuineness of all signatures, the authenticity of all documents submitted to
us as originals and the conformity to all authentic original documents of all
documents submitted to us as copies.
We have assumed that the Indenture has been duly authorized, executed
and delivered by the Trustee and constitutes a legal, valid and binding
agreement of the Trustee. In addition, we have assumed that there will be no
changes in applicable law between the date of this opinion and the delivery of
the Exchange Notes and the Exchange Guarantees. Based on the foregoing, it is
our opinion that:
1. The Exchange Notes have been duly authorized by the Company
and, when (a) the Registration Statement has been declared effective, (b) the
Exchange Notes have been duly executed, authenticated and delivered in
accordance with the terms of the Indenture, and (c) the Exchange Notes have
been issued and delivered against the exchange of the Existing Notes in
accordance with the terms set forth in the Prospectus included in the
Registration Statement, the Exchange Notes will constitute legal, valid and
binding obligations of the Company, enforceable against the Company in
accordance with their terms, subject to applicable bankruptcy, insolvency,
fraudulent conveyance, moratorium, reorganization or other similar laws
affecting creditors' rights generally or debtors' obligations generally,
general principles of equity (whether considered in a proceeding in equity or
at law) and an implied covenant of good faith and fair dealing.
2. The Exchange Guarantees have been duly authorized by the
respective Guarantors, and when (a) the Registration Statement has been
declared effective, (b) the Exchange Notes have been duly executed,
authenticated and delivered in accordance with the terms of the Indenture, and
(c) the Exchange Notes have been issued and delivered against the exchange of
the Existing Notes in accordance with the terms set forth in the Prospectus
included in the Registration Statement, the Exchange Guarantees will constitute
the legal, valid and binding obligations of the respective Guarantors,
enforceable against the respective Guarantors in accordance with their terms,
subject to applicable bankruptcy, insolvency, fraudulent conveyance,
moratorium, reorganization or other similar laws affecting creditors' rights
generally or debtors' obligations generally, general principles of equity
(whether considered in a proceeding in equity or at law) and an implied
covenant of good faith and fair dealing.
The opinions expressed herein are limited to the laws of the United
States of America, the state of New York, the Business Corporation Law of the
Commonwealth of Massachusetts, the Delaware General Corporation Law and the
New Jersey Business Corporation Act, and we express no opinion concerning any
other laws.
The opinions expressed herein are rendered for your benefit in
connection with the transaction contemplated herein. The opinions expressed
herein may not be used or relied upon by any other person, nor may this letter
or any copies hereof be furnished to a third party, filed with a governmental
agency, quoted, cited or otherwise referred to without our prior written
consent, except as noted below.
We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement, and to the use of our name in the Prospectus contained
therein under the caption
-2-
<Page>
Great Lakes Dredge & Dock Corporation
August __, 2001
Page 2
"Legal Matters." In giving such consent we do not thereby admit that we are in
the category of persons whose consent is required under Section 7 of the
Securities Act.
Very truly yours,
/s/ Dechert
-3-