We took the Kayak and catamaran out while at the Royal last week. It was nice and relaxing. I agree with Shammy though. not a whole lot to see and you have to stay in a certain area.
There was a very nice young man Mexico City who took us out on the catamaran names Alex. He will let you swim out there and snorkle a bit. Ask for him. He was a pleasure to talk with while out in the ocean.