Let me echo what Skittl said - DON'T do All-Inclusive on Cozumel. First, most AI resorts (with the exception of Cozumel Palace, one of the higher priced AIs) are way south - convenient to diving, but very far from Downtown, and unless you rent a car or spend a fortune on Taxis, you'll miss out on the cultural experience that is Cozumel, a very friendly place with wonderful people. Second, Cozumel has a LOT of great restaurants, and for every budget. At any of several little locally owned loncherias, you can get a great lunch, with chips and salsa, soup, an entree wit rice and beans, and agua fresca to drink, all for 70 Pesos (less than $5 at current exchange rates). Everyone has their own favorites, I like La Candela and Cha Cha's Kitchen, but compile a list from one of this Board's threads on favorite restaurants. There are also great seafood places for lunch, like Pescaderia San Carlos (my "must go" for a Saturday lunch, when they serve Sopa de Mariscos, a delicious seafood soup), and La Perlita, which has a 2-for-1 special on Lionfish on Thursdays (the coconut crusted Lionfish is great, and Lionfish Ceviche is very tasty). And for dinner, there are a ton of options, a light and inexpensive dinner at Del Sur Argentine Empanadas, a party atmosphere and great food at El Moro, Italian food or Pizza at Guido's, or Rolandi's, or New Especias, or any of several Taquerias back in town on Ave. 30. Want to splurge? Go higher end at Kinta, Kondesa, or La Cocay! I personally have so many favorites in Cozumel that I can't get to them all in a one week trip.
As for Diving, be careful about the hotel-dive op package deals, you may be getting less than you think. Personally, I won't go that route. I have a few favorite Dive Ops, and I always book the diving separately. Your choices may be limited by where you are staying and your budget, but there are quite a few that will give you valet service - rinse and store your gear between dives, have it all set up on the boat for your dives, and pick you up at a downtown pier or take you to the Caleta to meet your boat.
Finally, don't worry about diving Cozumel. The visibility is amazing, the coral formations at places like Palancar Caves and Palancar Bricks are awe inspiring, and the sea life is wonderful. Yes, it is drift diving, but I LOVE drift diving - it is the most relaxing way to go, the true lazy man's diving, if you remember that the current is your friend - don't fight it, just let it carry you where you want to go. Get a personal DM for your first day if you are nervous, and he'll show you the ropes on Cozumel drift diving. You'll love it.
EDIT - my comment on not doing the hotel-dive op package deal should not be interpreted to apply to the deal mentioned above with Tres Pelicanos - although I've never used Tres Pelicanos, it has an excellent reputation as a very good valet service Dive Op. Really, I was talking about various Dive Paradise package deals. Although Dive Paradise is a generally good Dive Op, it has two different levels of service - smaller faster boats that go to all the dive sites for the more experienced divers, and slower big boats that really only go to a few of the shallower and closer in dive sites. If you buy a package deal with Dive Paradise at a "very good price", you might find that you've only bought the second-tier service, and you'll either end up paying extra to get on the good boats, or you'll only see a very limited view of what Cozumel diving has to offer.