DC has a lot of surprises, not least of which if the floating fish market. At the end of L'Enfant Plaza, down on the Potomac River is a series of permanently moored but floating barges that contain an incredible floating fish market. There is a wide variety of fresh fish and shellfish available. The supply comes from far and wide, with an emphasis on freshness. Several vendors steam to order to take it home and fix it your way (or call and bring it out to condo and I will cook it.)
It is a little challenging to get to. The nearest Metro station is Smithsonian, about a mile away, down hill going to the market and uphill going back. You can also get there by car, parking is available but can be quite crowded. It is a great location to ride a bike to and from. Take a lock, the crowds can get tight and they are not very accommodating to bikes along the vendor rail.