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I dove with Oasis while on a Carnival cruise last year. I agree that the dives were conservative, but expected. I think I would do the same if I were the DM, knowing at least a couple of divers are going to crash into the reef.
I had a great time on both dives, nearly an hour each. We saw Hawksbill turtles, a couple of large Nassau Grouper, garden eels, trumpetfish, and much more. It might not be the best dives of your life, but I'm sure you'll enjoy every minute of it.
By the way, there was a nondiver on the boat with us. She snorkeled while we were in the water. There was also a deck hand tending the boat.
C-Eye diving, Blue Water and Oasis are all great dive services. Oasis serves the cruise boats.