Disclaimer - I have never been to cozumel. But I have been to ambergris in Belize and absolutely loved it. Their is a number of dives just off the coast of the island, but the main attraction is of course, the Blue Hole. We wen't on a day trip that included the Blue Hole and two wall dives. Some of the most unique diving I have done to date. A number of sharks, bat rays, groupers, couple of speckled mantas (4 and 6 feet).
We stayed at Ramon's Village which had some of the best accomodations on the island (from what we saw through exploring). Just make sure you get your room as close to the water as possible, since it is near the airport. Diving was a breeze, since they have their own dive shop.
I will make it to Cozumel one of these days.