I don't know how much we're talking about here, and if you trust the other person or not, but I just typed up my own simple little contract
saying I was going to give a certain amount of money
as a deposit towards the full purchase price
we had agreed upon, and the price would be paid upon the boat being put in the water
and the proof that she would float.
Granted, the amount I paid is a small amount compared to most boat purchases. And it was just a person to person deal, no broker involved. And the guy ended up being very trustworthy and nice.
Overall, the contract
ended up being about a paragraph and I just had both of us sign and date a copy for each of us to keep. A lawyer probably could have torn it to shreds, but it might have held up in small claims court(good enough to get my deposit back at least had something gone wrong)