@kd3pc - good point about the sewage treatment plant! We drove by there not long ago and decided that wherever we end up, marina or not, we needed to have quite a space between us and the plant. Phew!!!
@ All - I've done some outside research and it confirms most of the feedback here. There are limits, zones, and virtually every other variety of restriction on the Potomac anywhere near the DC area. To be honest, it was the possibility of the whole river freezing over that was the deal breaker for me. If I can't get out to cruise, then it's not the place for me!
We are going to direct our attention to the Chesapeake Bay (around and south of Annapolis) instead of the Potomac. Plenty of access to the ocean, plenty of marinas with liveaboard slips (..I hope!), and it doesn't freeze over...uh...right?