DCL has "sand-friendly" wheelchairs (with large balloon wheels to go over the sand easier) on Castaway Cay if the walking is difficult for you. And with the heat and size of the island, it could be. They are available on a first come, first serve basis but they seem to have a good supply.
The form Kathleen is talking about is called a Medical Clearance form and it is very important that the ship's crew know that you may require assistance "in the unlikely event of an emergency"
Have a Wonder-ful time!