You need an official form of boat title registration
. This is normally from the country where you are a citizen. The documentation of the transaction of the sale
may be part of the process for securing that document but on it's own means nothing. Some country needs to validate that you personally own this boat. A bill of sale
is alone not sufficient.
Other than that a valid passport is required for all persons aboard. Visas may or may not be required. Licenses for any VHF
/ HF radios would also be a very good idea. Not knowing the requirements would not be a good idea. You will always be presumed to have known. Without proper documentation it could be assumed you stole the boat. Should the situation come to that they can toss you in jail and they do not need to probve the boat is not yours. It's serious stuff.
The Documentation is proof in lieu of title
USCG documentation is title - always - any place. It is renewed annually at no cost - don't forget.