Make 2 stops:
First to the police department with the ID number to see if there were any reports of it being stolen. If no reports were filed then the second step is to stop in to the local Clerk of Courts or whatever office handles license and registration with the ID number you have from the boat which can be used to contact the last known owner. The state should have salvage laws, which you follow and use to your benefit stating you made reasonable attempts to contact the previous owner (sometimes as simple as a single certified letter).
It may be possible the person wants it back, in which case you can use the law to charge storage fees if you choose, and if they refuse to pay then they need to sign a paper refusing to pay and legally signing over all rights of the property to you.
Usually after a specific timeframe (30, 60 or 90 days) without any response from the owner, it legally becomes yours to do as you wish, but make sure you have copies of all legal paperwork for this situation on hand. Since it has been in your possession for 2 years now, it may already be legally yours and you just need to pay for a new title and registration, but check the law to make sure. Usually at that point you get a salvage title which essentially makes it yours and re-establishes a normal title in your name.
I have heard of a few cases where people came back months or years later to claim "their" property but since it already went through the salvage and legal system, they have zero power over it anymore... but for that same reason, keep equipment that may be stolen out of it when parked, keep a lock on the trailer hitch and another lock on the boat to trailer or post or something for at least 2 or 3 years just to be safe (if they have your home address or location of the boat).