A broker can do nothing that you cannot do (for example - they do not have any access to boat ownership documents) They simply add another link in the buying chain and there have been cases where the broker does a runner (or goes bust) and your money is gone. - both are not unknown.
A broker is under no legal requirement to inform you about any defects - they are acting on behalf of the seller.
As listed above, look for evidence that the boat belongs to the seller, and the seller is the owner, compare driving licence with the names and address on any boat documents.
The term Caveat Emptor was written to describe buying a boat.
What Does Caveat Emptor Mean?
Caveat emptor, a Latin phrase meaning "let the buyer beware," implies that buyers take the risk in a transaction. Potential buyers are warned by the phrase to do their research and ask pointed questions of the seller. The seller isn't responsible for problems that the buyer encounters with the product after the sale.