Here are two places I would highly, highly recommend for outstanding seafood in the DC area:
Black Salt
4883 MacArthur Blvd. NW
Washington, DC
202-342-9101
Menu
Pesce
2016 P St. NW
Washington, DC
202-466-3474
Menu
Pesce is much less expensive (~$40/person), but not quite as romantic or as good as Black Salt. Both restaurants take reservations, and both have an outstanding selection of unique, and fresh fish. Kinkead's might be do-able if you order several appetizers and split an entree.
Regardless of where you decide to go, I would call NOW to make sure you get a table. You wouldn't believe how fast restaurants book up downtown.