You do need to be careful how you register your boat. I live in PA and have a documented vessel that is registered and kept in Maryland. I pay a small amount for a Maryland use sticker every two years. I also paid sales tax to the state of Maryland when I bought it.
The state of PA came after me a year or so later wanting a 6% "Use Fee" plus penalty and interest.
I had to prove to them with letters from the marinas in MD and my insurance paperwork showing where I kept it. I did not have to pay the Fee, but they warned me that if the boat was ever in the state of PA for more than 24 hours they would come after me again.
That warning was from the tax people stating the 24 hour part, but I could never find that in writing elsewhere. The boat registration paperwork says you owe it after 30 days. I think the issue is residency.
They did not care what amount of tax I had paid to the state of MD.
This is what PA says about taxes paid to another jurisdiction (taken from the back of the application). The sticking point for the exemption is your state of residence.
7. Boat purchased out of state and registered, by a nonresident applicant for registration solely for use in Pennsylvania as a tourist or vacationer. (Exemption is not permitted if applicant is a resident of Pennsylvania or it boat was originally purchased in Pennsylvania.)
Now, here is the part I would be worried about. Maryland does not recognize taxes paid to jurisdictions that don't reciprocate. This is a quote from the Maryland regulations.
§8-716(f) of the State Boat Act allows reciprocity credit for excise or sales tax previously paid to another jurisdiction if:
The vessel was formerly titled or numbered in another jurisdiction; or
The vessel was formerly federally documented and principally used in another jurisdiction; and
The present owner paid a sales or excise tax on the vessel to the other jurisdiction (proof required – validated receipt, etc.); and
The jurisdiction to which the tax was paid would allow an equivalent exemption or credit for vessel excise tax formerly paid to the State of Maryland.
That last item might keep you from getting credit for PAs tax monies when you go to MD. I would get something in writing before I made any assumptions.
I just took the easy way out an vowed to keep out of PA. The Delaware River is lousy sailing compared to the Chesapeake anyway. If you are buying an expensive boat, not getting this right could cost you a lot of money.
Don't forget that the same thing applies to your dingy if you use a motor on it. Maryland is big on enforcement of dingy registration.