You should consider using an escrow service
for handling deposits. But shop carefully and only get the service
you want. The local one in my town is basically too expensive for what they do but the one in Seattle
did a great job and it makes getting the deposit back much easier. The first boat we bought we did DIY and sent the deposit to a guy in another country (Canada). Fortunately it all went well but whew!! I was sweating that deal.
But you should be able to use a boiler plate contract. Just use the "subject to your approval of a professional survey and your OWN survey". Basically you should make sure you can opt out for any reason whatsoever up until a certain point - and get all your deposit back. But you normally will pay for the survey, haulout, etc. even if the deal falls through.
Make sure the guy actually owns the boat free and clear though. Escrow companies can also help with that if you want (no liens, registration