Here in Florida a Bill of sale along with a form from the DMV works as long as the boat has never been registered. They looked at the VIN and made sure it came back legit and they titled it no problem at all.
I bought a pond jumper boat that was only used on a private lak and never was registered so it was no problem at all.
They told me if it had been registered before that it would be alot harder and costly to get a title for it. They have to try and make contact with the previous owner to verify it was sold and not stolen. If they get no response then they would free up the title and you could apply for it. But it did cost more, Just not sure how much.