ok, you can see what I do in my profile but I'm a civil litigater not a criminal lawyer, this post does not constitute legal advice.
I agree to an extent with the caution about simply taking it as plod may well nick you until they can sort out what has happened. .
a buyer cannot obtain good title in something the seller did not have title to sell. There are probably procedural steps you can take in the admiralty court to 'seize' the boat. PM me and I can suggest a good shipping lawyer who will probably be able to help you for a fixed fee if you ask him nicely. It'll be worth a few hundred quid to do it properly if the boat has a significant value.
THIS POST DOES NOT CONSTITUTE PROFESSIONAL ADVICE AND YOU SHOULD SEEK INDEPENDANT LEGAL ADVICE FROM A SOLICITOR.