The best diving is on the Palancar Reef.  The Palancar Reef is located about 10 to 15 minutes offshore, if you are staying at the Iberostar or Allegro Hotel.  If you are staying at one of the hotels south of town while you could dive, near them, the diving is not really great.  For the great diving in Cozumel you have to take the boat and go to where the good reef’s start.  Those 'good reef's" start 'in front' of the Iberostar.