If your residence is in Mass. couldn't they trace you through your documentation numbers?
In Rhode Island they called that a personal property tax that was based on the value of the car/boat. We had the California registration numbers still on the boat and CA plate on the trailer, weren't in RI long enough to bother to change it over so were not dinged with boat tax. We did have to pay the PPT on both cars though.
In PA it doesn't matter if your boat is documented or not, you still have to register it in PA and have at least a PA registration sticker displayed.