As you know, Coz is not a shore diving destination, but here's a few sites:
1. Nice reef & sunken barge in front of El Presidente . On site shop, Scuba-Du, will rent you stuff. You'll be stopped by a guard in the driveway (very chi-chi hotel)--make up a room number, like 78.
2. Broken up small airplane in front of El Cid/La Cieba. On site shop, Aqua Safari, will rent you stuff.
3. Some nice patch reef, e.g., Pico's Reef, off Villablanca Hotel. Near to Papa Hog's Scuba Emporium which will rent you stuff.
4. Trash dump off Barracuda Hotel is not scenic but loaded with marine life. Serviced by Dive Paradise, they will rent you stuff.
5. Chankanaab Marine Park has a modest entry fee & some nice patch reef. Rent or bring your own gear; must have a DM.
6. Paradise Reef across from Villa Dzul Ha (between Presidente & Chankanab). Hog's is about 1/2 mile north for rentals.
7. Nice reef off the northern hotels, e.g., El Cozumeleno. Pepe's & Scuba Du up that way will rent tanks.
Watch out for boat traffic.
Best regards.
DocVikingo