In my experience an original death certificate will be needed to transfer the boat into your name. I assume you have registered the boat? How is it worded? There are two different wordings that can designate transfer ability here. I'll give both examples:
Name/s on title:
Andrew Smith AND Betty Jones
Andrew Smith OR Betty Jones
The latter allows for one of the parties to transfer title without the notice or signature of the other
The former requires signatures of both designated by the inclusion "AND".
If both are required and one is deceased an original certificate of death ( available many times online) can be required for the surviving party to take sole ownership
As suggested many times here, find an attorney, legal aid service