FULTON'S!!!!!
That's real seafood!
Narcoossee's is good, but a little limited in choice and it is a 2 credit restaurant.
Flying Fish has some seafood dishes, but again, 2 credits and somewhat limited in scope, though these two are favorites of mine.
Also worth a look is Todd English's bluezoo at the Dolphin.
Keep in mind that neither of these restaurants is on the dining plan.
If you want sushi, go to Tokyo Dining or California Grill. I think there's also still a sushi bar in the Swan, but I've never been there.
Don't waste your time on either Coral Reef or Cap'n Jack's, their selections are too limited.
As always, check the menus at All Ears:
Restaurant Dining Menus from Walt Disney World