National Harbor, Washington Sailing Marina, Gangplank, and James Creek are your choices if you want to be close to DC.
A friend of mine kept (and sailed) his 42' Catalina at Gangplank for over a year. Gangplank is the farthest North of the sailing marinas and the closest to a Metro station (L'Enfant Plaza). It is also pretty expensive.
James Creek is next to National Defense University and the USCG HQ building, is pretty cheap, but isn't much to look at.
Both James Creek and Gangplank are not in the best neighborhood (Southern DC is pretty blah, neighborhood-wise) but are plenty safe in the immediate vicinity of the marinas themselves.
National Harbor is beautiful, clean, new, and expensive, not close to any metro stations, though.
Washington Sailing Marina in Alexandria is nice, has a good restaurant and is (long) walkable to other dining options, but is still over a mile to a metro station.
I kept my boat at James Creek for over a year and was pleased enough. There's not much community there, and there are more stinkpots than sailboats, but I got what I needed for cheap. It is possible to walk to a metro station from James Creek, but I can't say I'd recommend it, as you end up walking through an industrial area close to the stadium and some questionably safe neighborhoods.
Hope this helps.