Sorry about the delay, I have being traveling a lot. But I hope the answer is still usefull. Playa is a good place for pass the nights but Cozumel is the right place for have a great diving. You have to take the Ferry from Playa del Carmen to Cozumel do not get a diving tour from a Scuba Diving shop in Playa del Carmen, wait to pass to Cozumel, you can find many shops just few blocks from the Ferry port. The best place to dive is the Devil Throut or the Palancar recif, this is a wall recif and is around 90 ft deep. If you want a less deep recif Santa Barbara is a good choise, there are many place to enjoy, the guide from the Dive Shop can recomend you the best for this season. You can contact the Dive Shops usin internet with the key word Dive Shop Cozumel. I expect the information can be usefull. Best.