Boats are titled differently — the paperwork matters more
Depending on the state, a boat may be titled, registered, or both — and outboard motors and trailers are sometimes documented separately. A bill of sale carrying the hull identification number (HIN), length, and builder is the backbone document every registration office asks for.
What to include for a vessel
The HIN (usually stamped on the transom's starboard side), year, builder, model, and length; the registration or title number if the boat has one; and whether the motor and trailer are included in the price. If they are, list them explicitly in the description.
Selling motor and trailer together?
Switch the generator to 'Other personal property' and describe all three items in one document, or produce separate bills of sale — some states require a separate one for the trailer since it's registered with the DMV rather than the wildlife or boating agency.