Never with a boat but I have sold quite a few cars.
You need a simple legal document stating what will be delivered by who and when. Do a google for bill of sale. There should be some available for less than $20. Depending on how much your boat sells for you may want to have a lawyer review whatever you put together.
The basics .. make sure the B.O.S. states that the boat is being sold "AS IS" and you will not be responsible for any damages (direct or incidental) to the buyer as a result of the boat.
He should pay you will certified funds (cashier check from a big bank or wire transfer) when the boat is delivered. It is then on your shoulders to pay off your note and send him the title within X days. All that will be spelled out in the B.O.S.
That should get you started.