Better yet, IMO, would be to get completely certified elsewhere before the trip. For one thing, the newbies could spend the entire vacation diving the reefs. For another, conditions around Cozumel are near best case, so certing there gives a new diver a narrower comfort zone.Do your class and pool work at home and do the open water cert on Cozumel. That way you can get a lot more dives in during your vacation.