Its a funny thing, but most states have a "department of motor vehicles" and a "tax department" and whatever they are called, they usually can be found on the web. They also usually post FAQs that tell you what is subject to registration and tax, and usually treat a boat as a "motor vehicle" as long as there is a primary or auxiliary engine permanently installed on it...
I've lived in several states and registered boats in two, so I'm somewhat familiar. NJ does boats through DMV. PA has a separate Fish & Boat Commission. MD seems to use DNR. (Some of my terminology may not be exactly correct.)
Ideally it should be as simple as you say it is. But PA has a bad reputation of refusing reciprocity with many surrounding states, and this is a reason why it is so muddy. Like someone else mentioned, Maryland law seems to offer reciprocity to states that offer reciprocity to them. That may seem to be a "finger in the eye" of Pennsylvania residents. And Maryland also seems to be a bit hostile because Delaware is such a magnet for big ticket items because they have no sales tax. So MD ramps up their enforcement against everyone, but mainly because they want to pick up people who use DL as a shelter but actually use their boats in MD.