There are a number of marinas on the Anacostia and further down the Potomac (Occaquan, Aquia Creek, Ft. Washington, Columbia, etc.).
But it depends on where you intend to boat, what kind of boat you have, and what you do with it. From DC by boat it is about 200 miles to Annapolis (down the Potomac and back up the Bay).
The Bay is more interesting, but requires more time to take advantage of it. For us, if we only have a limited time, we cruise the Potomac (day trip, overnight). Longer trips are on the Bay (3 days to 1 week+). But my boat is small and can be repositioned via a trailer at 60 mph.