Any suggestions on how to protect your deposit money if you have to
say "no thanks"?

Make the purchase subject to a survey of the vessel just prior to paying the
balance.
(Very good idea, any way).

Include language in the contract that failure to deliver a vessel essentially
identical to the boat displayed as a sample (or without structural defects)
will be considered a material and incurable breach of the contract.

Where are you seeing a steady stream of
crappy GB's?