I've never seen them onsite, but nowhere really specialises in seafood on property at Universal..........I don't count Bubba Gumps as a proper seafood restaurant.
Offsite there are several options.......BigFin seafood being one, Ocean Prime, Lee and Rick's Oyster Bar.........Oceannaire is another nice one.
Further out is High Tide Harry's...........fabulous place to eat oysters.