Just wanted to report on the fun time we had this past Saturday night. We went to the Marina (Waterfront area? Water Street SW?) and there were all these places selling raw or cooked fish and shellfish (some had produce too). All of them had fresh steamed or boiled shrimp, many had fresh steamed crabs (Carolina (blue?) crabs "Our Crabs Have No Sand!" - the live ones we saw did have a blue claw) a few had vegetable crab soup or clam chowder, crab cakes, crawfish, steamed corn, hushpuppies, etc. Many had Alaskan crab legs (frozen and re-heated).
There aren't too many places to sit, we saw lots of people get large tubs of steamed crabs to take home. There were people eating crabs there, though!
Most of the places close at 9, a few - clearly the more popular ones - closed earlier because they ran out of food. It's close to the tidal basin, so maybe a lot of people went there after seeing the cherry blossoms.
Anyway, it's a fun thing to do :)