Correctly this is called documentation
and is managed by a department in the US Coast Guard. I believe there is a form to do this but if you call I have found them very helpful.
From the web site.
HOW DO I DELETE A VESSEL FROM DOCUMENTATION?
A vessel may be deleted from documentation upon the request of the vessel owner or an authorized agent on behalf of the owner. The request must be in writing and is to include the vessel’s official number. The original Certificate of Documentation (COD) is to be returned by the owner named on the certificate.
Hours of Operation: 7:30 a.m. to 5:00 p.m.
Desk Hours: 9:00 a.m. to 3:00 p.m.
, and Phone
USCG/National Vessel Documentation Center
792 T J Jackson Drive
Falling Waters, WV 25419
Contact NVDC via E-Mail
E-Mail address is email@example.com