I would not recommend the shipwreck snorkeling in Grand Cayman. We just did that in February and there wasn't much to see. No coral at all just a few fish, the water wasn't vvery clear. I would snorkel in Cozumel, I think it is much better!
Last trip to Caymans we did a tour through the ship that combined snorkellig on the reef
that surrounds the island with a trip to sting ray city. The snorkelling was excellent. We anchored in about 20 feet on a very sandy bottom. Rays and fish swam below us. As you swam towards the reef. coral color picked up and fish were everywhere. We encountered (with advance warning) a moray eel and a six foot sand shark. After about 90 minutes we moved off to do the sting rays in shallow water.
Catch the Wave in Cozumel is a trip I have done. Nice boat, friendly crew. Decent snorkelling . The reef is only 1/4 mile beyond the cruise pier that is outside town.
There is a small glass bottom boat carried aboard and they offer free rides in that as well. I recall good food and a nice party with adult beverages on the ride back in.