“I imagine that a lot of Maryland folks with boats...will find other states to register their boats and simply keep them parked in those states”
Good observation. In fact, the number of boats registered in Maryland has already decreased going from 205,000 in 2007 to 193,000 in 2010.
If you keep a boat in a Marina on the Potomac, you have to register it in MD. MD owns the river up to the mean high tide mark on the VA side. So, except for Columbia Island, which is actually in DC, this is also a tax on VA boaters.