I'm not a lawyer, but I have caught some sharks! Anyway, you need a contract/lien on the boat, just like a bank would have. Make up a bill of sale with the payment terms and the requirement that the new owner has insurance. Both parties need to sign and get it notarized. The new owner can take the bill of sale to their insurance agent and get insurance. You can release the title when payment is complete.
The path to hell is paved with good intentions.
Marc, seen your post over on THT, a lot of legal (and GOOD) types over there who, from years of reading some of their posts, actually give good advice...So, that being said, how good of a "friend" is this friend? How long have you known him? What's your true gut opinion of him? In other words, do you trust him to make good on his end of the bargain?