Mike, there are a lot of them to choose from. I would suggest that if you have a local dive shop near your home, go there and ask them who they would suggest. You could also do an ask.com or google search
They do Sting Ray City & snorkling. Established in Grand Cayman in 1951, is one of the original operators of snorkel trips to the Stingray City sandbar in the clear, warm waters of the famous North Sound.
While Captain Marvin's is a well established snorkel tour operator, I do not belive they conduct scuba diving trips.
Shore diving is available at Sunset House, Eden Rock, Don Foster's, and DiveTech.
Many small dive operators can adapt schedules to fit cruise ships guests. Check with Ambassador Divers, Neptunes Divers, Wall to Wall, and others.
Finally, do a little research on dive specific websites such as Scubaboard. Be sure to use the search function as these questions have been asked hundreds of times!