I just like the plain Grouper sandwich at any of the Royal Resorts' restaurants - you can also order one poolside. Fried, grilled, whatever way you want.
Just about anywhere, you can order a grouper fillet, again, cooked your way.
They often have Grouper on buffets cooked in tomato etc, but I think that preparation overcooks the fish and has too many competing flavors.
If you're in to deep sea fishing, you can arrange that through the tour desk in the resort, and when you get back with your fish, give it to the resort's restaurant - they'll cook it however you want, and serve it with rice or potato, vegetables, salad, for something like $6 per person (which may have gone up).