Alexandria has a marina, and there's a new National Harbor project going in near Fort Washington, MD, which is supposed to have some marina berths. Both might be worth checking out.
A potential downside to the National Harbor Marina is that I believe it's just downstream from the DC sewage treatment facility. Not saying that there's a pollution problem there, just that when the wind is right you may get a powerful aroma. I used to work north of the plant and there were days I wished I could turn off my nose. But, maybe it's far enough away that it's not a problem.