Julie,
So exciting! First trip to WDW! You are going to have a great time!
So opinions vary (as they are wont to do!), but my favorite seafood is at the
Flying Fish at the BoardWalk. It is not cheap, but the seafood quality is exceptional, and the cuisine is inventive and delicious.
Plus you get to walk out onto the evening atmosphere of the BoardWalk, which is a nice "dessert" on it's own.
A good second choice, and in a more reasonable price range, is
Coral Reef, located at the Living Seas pavilion in Epcot. The view into the attraction's tank is impressive.
Now if it is sushi you are seeking out, there is
Kimono's in the nearby Swan, but the very best sushi on property is actually at the
California Grill, at the top of the Contemporary.
Please feel free to send me any follow-up questions!
Have a magical (and delicious) vacation!