I have done the scuba in Cozumel several times. I have been diving all over the Caribean so I can give you a good answer to your question. The dive in Cozumel is one of the best. It is what is called drift diving. The current takes you about a mile where you are picked up by a boat. They water is fairly clear and there are lots of fish.
I would recomend taking the Carnival tour. There will take you to a safe dive where you will have aplace to put your valuables.
One time we were delayed at the dive site and the dive boat took us directely to the ship. Carnival wants you to be safe on your dive and the people who run it are very good. Enjoy