You need a bill of sale to pay taxes on and register the boat. The person getting the car needs one for the same purpose. You need a copy of the car's bill of sale to prove, if you ever needed to, that you sold that car on a certain date (before some dude you never met before and found on FB or CL drives it drunk into a pedestrian and drives away). He might ask for a copy of the boat's bill of sale for the same purpose. Helps to have a home printer scanner but you can just write out two copies and sign them both, or take pictures with a phone or digital camera (which is helpful because it will have date/time data to go with the picture).