The rule is this: Buyer's price, Seller's terms(i.e. cash)...... Buyer's terms(i.e. installments), Seller's price.
Negotiate on one or the other but not both. So only a full price offer will warrant a consideration for installments. Like a vehicle, you'd put your name and address as the lienholder on the registration. This allows you to repossess the vessel if he defaults. You can draw up a simple contract regarding payment dates, insurance, slip and maintenance fees, boat location, etc.
You can get a better price by offering installments.... I purchased a boat that way and spent the rest of my budget on improvements.... after payoff the boat become unencumbered, lienholder removed from title. I would not have defaulted after spending so much on new sails, motor etc. so my seller felt pretty secure that if he had to take the boat back he'd actually make money.
This purchase was for well under $10k.... for more expensive purchases I'd use a professional.