Hey folks - yesterday I hopped on the Metro, rode it 2 stops to the Smithsonian station, then walked south on 12th St about 10 minutes to get to the wharf on Maine Ave. Previous Chowhound posts have discussed the fresh seafood offered there, but I indulged in a cooked lunch (altho with the heat & the lack of a clean place to sit, I had to wait to chow down back at my desk)
3 barges/stands offer cooked seafood - Jessie's, Cptn White's & the Maine Ave. Seafood Co. Price are about the same at all 3 - fish sandwiches start at $5, crabcake or soft-shell crab platters (incl. fries, hush puppies & slaw) end up at $10. You can get crab soup for $4 a bowl.
I had a delicious set of 2 soft-shell crabs, only lightly fried (& done right there, not sitting under a heat lamp), plump, yummy - best I've had in a while! The hushpuppies were OK but I could have done without the pile of greasy steak fries. There are a bank of Coke machines nearby.
Not sure if the walk from L'Enfant Plaza is any closer, but the walk from Smithsonian is more direct & it's 1 stop closer to my office.
Added bonus on Fridays (June thru Oct.) from 10-2: the farmer's market at the nearby USDA, right near the metro station.