I know this is an old post, but in case you are still out there... you can anchor
just abut anywhere for as long as you like as long as it is not specifically prohibited. Common sense should prevail... don't anchor under a bridge or near any facility that might be under the watchful eye of Homeland Security
. I believe DC waters (anything above the Woodrow Wilson Bridge) are prohibited for long term anchorage, although I have had no problems with overnights.
Tantallon Marina is filling up fast and has a few slips left. I know, it has it's pain-in-the-ass factor, but if you treat the owners with respect, you will get a fair shake. AND there is none of the snobby crap you find at some of the "tonier" marinas
. So, bring what you got... as long as its insured... and you'll fit right in.