I wish Disney had a restaurant with a menu with various choices of shell fish - but I haven't found one yet. Mostly you are limited to lobster at upscale restaurants and some restaurants offer crab cakes (mostly as appetizers). Flying Fish is not a shellfish restaurant IMO. The clambake offers steamed clams, peel your own shrimp and mussels with crab legs as an add on for a price. Captain Jacks has crab. I wish Disney would have what I call a seafood restaurant with a true variety of shellfish items.
You might want to check out the menus at www.allearsnet.com to see if anything tickles your fancy.
I had really good crab legs at Fulton's.
Like someone else mentioned there isn't one restaurant that has a great selection of shellfish. It depends what you want.