The owner should have a bunch of receipts for things they have done on the boat - surveys, blacking, repairs etc. Do they have any of this?
Ideally they will also have a receipt from the builder or the previous owner - if a previous owner, is there any contact info you could confirm with them?
You could look at using a solicitor to form an escrow agreement, where they hold your money in trust and release it when you are happy - but I suspect the seller won't be happy with any delay to them getting their money.
Buying a second hand boat is similar to a 2nd hand car - buyer beware at all times!