This year for spring break I went on a cruise (Carnival) with some friends. I was the only diver, and I looked at the official cruise line diving excursions, but after recommendations from others, booked my own diving with a smaller op in Coz. (Scuba with Allison) It was easy to get from the cruise dock to the dock I met the dive op at, maybe 5 minutes in a taxi. It was cheaper than the official cruise diving, as well, and there were only 6 of us on the dive boat instead of what I would imagine to be a larger crowd with the cruise line's choice.
FWIW, on our stop in Grand Cay, my friends and I had booked a snorkel expedition and when we got off the boat, we found out it had been canceled, as had a few other (non-snorkel) excursions that morning. I wouldn't want that to happen with my diving.