Hi rrhara, welcome to Cruise-Chat. We have done a few snorkeling trips through the ship, most of them have been near to where you tender. I think there are a couple of dive shops that will let you use their docks/ladders, but not positive. We did 3 different wrecks. The water was great, very clear. The last trip we did the Stingray City trip again. This trip we're going to either 7 mile or Rum Point.
If you'll use your "Find" feature in the upper left hand corner and type in your port, it will bring up all the posts made about Grand Cayman. You might find an excursion that interests you. You might also like to browse through the Caribbean Port Chat section. I can honestly say, the ship tours I've booked, I've not been disappointed it.