Just had the treat of dining at the American Sea Grill - DC has excellent luck with hotel restaurants. While the fair wasn't terribly adventurous, I would be pleased to go out of my way for their Blue Crab Soup - it's not a bisque, it's a dark broth. Very flavorful, huge lumps of crab, gruyere cheese on top. It's everything you love crab and french onion soup in one.
Not too unbearably priced, either.
If you're in L'Enfant Plaza anyway, I would definetly go.