although Lauderboy has a great idea,
I know there are no marinas commuting distance to DC that offer that service. Even fewer for liveaboards. And fewer yet that have pump out at the slip like FL marinas often do...
Even being in the rivers for more than a prescribed period of time is going to draw the attention of DHS...too risky for them to not bother you.
Good luck, annapolis is about as close as you will get for these types of services, and then they are $$$ and often have bridges to get under.