Gangplank, unfortunately, has a waiting list unless you buy a boat that comes with a slip there (or a slip that comes with a boat?) There's also Tantallon (Ft Washington) which has less of a liveaboard community and fewer amenities but may not have a wait.
Depending on your work hours and how many times you have to do the commute, you may look farther: non-rush hours I can routinely make Annapolis-Herndon in about 75 minutes (not speeding!). Also, Metro is being extended out to IAD, so that may give you another option. And from Annapolis
, from the time you leave your slip to the time you're sailing the Bay is about 10 minutes.