Holiday Point Marina is in Selby Bay (Edgewater, MD), near the mouth and on the south side of the South River. Lots of sailboats over there. Decent docks, several marine
services on site. I have no first-hand experience with 'em, though. They've got a website.
We're next door, but don't have the necessary depth
Selby Bay Marina, just slightly north of Holiday Point, may have some sailors; I know they have several covered sheds with powerboats, but I think they have some open slips, too.
Pier 7 Marina is also on the South River and the Rt 2 (Solomons Island Rd) bridge, across from Liberty (mentioned above), and there are several sailboats there... and there's a catamaran
broker/dealer of some sort there, too.
The South River Bridge clearance is decent, so depending on your mast
height, you could also maybe look at a couple of the marina's on the northeast side of the bridge.
Check Active Captain
and some satellite
or Google Earth
imagery for additional possibilities in the area (Pocohontas Creek, etc.).