Here is the trick with the Gangplank. If you buy a boat that is for sale there. The slip is grandfathered in. I actually know a gal who bought a crappy house boat there just to get the slip. Within a few weeks, she brought in her boat and moved aboard. That is why the list is so nutty there!
Almost all of the Marinas in Annapolis allow liveaboards now. In fact, all of the Marinas in Annapolis are openly advertising their amenities for liveaboards. Herrington Harbor North in Deale allows liveaboards. That is about 40 minutes from DC and is very nice. Expect to pay $200 per month liveaboard fee in addition to the slip fees on the Bay.