Ange and myself have just come back from Cozumel off the coast of Mexico. We booked through Regal Dive. You fly into Cancun. We flew with Thomas Cook airlines. Be aware that unlike commercial flights, most of what would be free on board, such as food, drink, films, you have to pay for. Because we had booked with Regal all transfers are included and arranged. Just as well. It is a 45minute taxi to Playa de Carmen, then a 45minute ferry to San Miguel de Cozumel and another 20 minute taxi to the resort complex which in our case was the Occidental Allegro. The journey back was just as well organised with a bonus of a Jazz/Reggee band on the ferry. All inclusive is recommended and the resort was very nice (although the American accent quickly wears thin).
The dive centre is at the resort and is operated by ProDive, Mexico. There is a weekly schedule on a board in the shop from which you choose either mornings or afternoons, two dives on each trip, you are not allowed to do all four. Night dives are available, at extra cost, on Mondays, Wednesdays and Fridays. We had four different guides only one was a disappointment. They are very knowledgeable and very accommodating for any quirks you may have. The dives themselves are generally swim throughs, a bit like the Canyons if you have dived with us in Menorca, but on a larger scale. When you come onto the top of the reef, as all dives do, then it is a drift dive. The main difference is the speed of the drift. I can say that if you are a photographer, it is best to spot the creature early and drop to it because swimming back to it isn't always easy.
The wildlife is very good although you are quickly aware that coral fish are basically the same the world over and the only things that change are the colours and the quantity. The creatures that are different are the big ones. Here, just about every dive you will get at least one turtle. Generally friendly unless you mess with them, some will even come to say hello. The other large creature of quantity is the Nurse Shark. Generally asleep during the day and they are not any risk to humans, unless you pee them off.
Am I glad we went, yes, would I go back, probably not. Recommended, definitely yes.