Boats get registered and/or titled in the State that you use them the most--not where you live. So if you use your boat in PA more than your home or any other State, then you must register it in Pennsylvania.
What boats need to be registered?
Do you use a motor?
Boats may or may not be titled depending upon the State, year of the boat, length of the boat.
In Pennsylvania, you will know if the boat is titled if you look at the registration card. There is a section on the registration card for a title number; if that section has all asterisks then the boat is not title and only registered. In this case the registration card is used to transfer ownership. If there are numbers in that section, then the boat is titled and must remain titled. In order to change ownership on a titled boat, the title must be notarized and transferred properly to the new owner--just like a motor vehicle.
As for other State's, please check with that State directly for what proof of ownership is needed for the sale of the boat.