You are going to need to have a good bill of sale.
Some generic sales slip for a receipt will usually not do at all.
If you want to be sure of the deal, you need a boat bill of sale that will clearly state the details of the vessel that you are buying or selling.
You can't register a boat with a motor without a proper bill of sale.
Motor means an electric motor or a fueled motor.
Buy a boat and get a receipt.
Call the RMV first to be sure what you need for your state as the rules change from state to state.
A good boat bill of sale should mention year, make, model number body style, VIN or the HIN (Hull Identification Number), and engine numbers.
Homemade boats or boats before 1972 without a hull number (Mass and other states law) often must be inspected by the Environmental Police.
If the law applies in your state then they will inspect the boat and apply a state assigned HIN tag on the boat and give the owner of the boat an inspection certificate for registration use later.
You have to show the Department of Revenue that you paid sales tax on the materials to build the boat.
Most States Require...
Most states require a small boat with an outboard motor to be registered even if you only use the motor occasionally.
I had a small dingy that was 8 ft long.
I used it to get from my larger sailboat to shore.
Both boats were registered and the serial numbers from the boats were asked for.
Massachusetts tells us that "Any boat powered by an electric or gas motor must be state registered. Only ship lifeboats are exempt from registration."
In some states such as Massachusetts, new boats getting registered require an original manufacturer's statement of origin (MSO) and it is used and provided as proof of ownership.
You can use an MSO, title, or previous registration as proof of ownership for used boats, as well.
Don't forget to apply the registration numbers at least 3 inches tall and in a contrasting color to each side of the front of the hull of the boat.