I just wanted to post a follow-up here. I can't recommend diving through Club Med Cancun and Scuba World International, although I didn't even get in the water with them. SWI is the exclusive dive shop to CM.
I don't know whether to blame the dive shop, or Club Med, but I had made arrangements to dive with the shop, Scuba World International, and they cancelled the trip the night before due to low signup (despite pressuring me that the trip would fill up earlier in the week). I was never notified of the cancellation, and both the dive shop AND Club Med claimed it was not their responsibility to notify me. They did not apologize, and I wasted a morning of my vacation trying to work out what happened and getting up early and preparing for a trip that didn't happen. They offered me very limited assistance in finding another shop and nothing, not even an apology for my troubles. It was a real disappointment.