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).

Two bills of sale is the way i would go

When you fill out the sales tax paperwork there is an "allowance for trade" and according to the town hall people you can do this with private parties, not just dealers.

I would just do bills of sale for $500 for each thing, for simplicity. Or you can do the $500 BOS but also do a more elaborate one describing the trades, and if they reject the elaborate one, go with the normal one.

Sell both to each other for 1 dollar Save the turtles

Nah just get the boat then immediately sail it to international waters. No registration required. Yar, harr fiddle dee dee enjoy your life on the open sea!