The best restaurant at SeaWorld (and its only sit-down restaurant) is Bimini Bay, right next to where the Luau takes place. Unfortunately, recently they have cut back considerably on their menu, offering just sandwiches and the like.
If they go back to the full menu, I highly recommend the mahi-mahi with pina colada sauce.
Of course, the park closes at about 6 and the luau is an evening meal.