Can't help with booking thru the cruise company, but, having been to Cayman and Stingray City, I can say you won't be snorkeling at Stingray City, every tour we looked at included snorkeling, but they take you to a separate area. Our hotel in Cayman used a company called something like RedSail, but the cost was crazy, we were able to google a few other companys that offered this for a fraction of the cost. We used Marvins, but I read great things about other companys and I bet most of them likely have pickups near the port. We had two snorkeling stops included, one at the reef. Both were the best snorkeling experience ever. My kids brought their snorkels and masks to stingray city thinking we could snorkel there, but they actually back the boats up to a sandbar where you can reach (little ones will need to be carried). They instruct you NOT to swim at all, but to keep your feet on the ground and shuffle along so you don't kick the stingrays. They are like cats rubbing against your ankles as you try to walk. (kids can have masks to still put faces in water, but it's shallow and clear, so not needed).