I wouldn't expect to find particularly high-quality seafood at a buffet. The serving style doesn't exactly lend itself to fresh fish, which typically requires precise preparation and delicate care. For a wonderful seafood dinner on Disney property, I recommend Flying Fish Cafe.
If you're looking for a buffet with seafood offerings, your options are pretty limited. Cape May Cafe is the obvious choice. It's a nice restaurant with a nice array of dishes on the buffet line. The shellfish is generally better than the fish, and the clam chowder is passable. The dessert bar is one of the best among the Disney buffets.
