My personal favorite Dive Sites: Palancar Bricks/Palancar Caves - Amazing coral structures, lots of sea life, some great swim-throughs, including a swim-thru near the beginning of Palancar Bricks that takes you down to about 85-90 feet and exits right over the wall; Columbia Deep; Columbia Shallows - a shallow dive, but beautiful, teeming with life; Tormentos - the current can take you on a wild, thrilling ride. There really aren't any bad dive sites on Cozumel - trust your DMs, they'll know which sites are going to be good on a given day.
Favorite restaurants: Higher-end places for great fine dining experiences - La Cocay, Kinta, Kondesa. Mid-range places, more touristy, but very good food - Guido's, New Especias [Italian Cuisine], Casa Mission (I love the gardens), Del Sur Argentine Empanadas - you can have a great light dinner here with just a few savory empanadas and a few dessert empanadas, Pancho's Backyard; MUST GO TO Local places - El Moro - a back-in-the-hoods hole in the wall that is just a ton of fun on weekend nights, party atmosphere with excellent food, also my go-to place for Sunday lunch when it has Cochinita Pibil, a real Yucatan treat, Pescaderia San Carlos (open only for lunch, I think it closes around 4:00 or 5:00), another hole in the wall place frequented mostly by locals, excellent fresh seafood, La Perlita (open lunch and dinner, has 2-for-1 Lionfish special on Thursday afternoon - must try the coconut crusted Lionfish!), Casa Cuzamil (sentamental favorite, this is the sucessor to Sabores, for many years a favorite Loncheria, now closed), Cha Cha's Kitchen, a loncheria with amazing cakes.