I also vote for the Presidente.  Even when they don't have advertised
specials, I call up the hotel directly in Mexico and cut a deal.
Although the Presidente is not an all inclusive, it has a great
breakfast buffet (which you can often get included in your room rate),
and it is by far (in my opinion) one of the better, if not the best,
upscale resorts on the island.  OK shore diving - there is a shallow
wreck right off the hotel.  Plus, there are so many reasonable
restaurants in town, there is no reason to do an ai (unless you have
small kids).

The hotel's website is www.cozumel.interconti.com.

Enjoy.

Ken

I would second everything that Gordon has said.  You can do night dives
from the shore from many hotels, but you would probably do better from a
boat.  Shore diving in Cozumel is OK, but boat/drift diving is much
better.  You've gone all the way to Cozumel, so find a boat and go diving.

My biggest complaint about the AIs is that their food is not very
interesting.  In other words, bland.  Why did I go to Mexico for that?
If you do AI, consider a couple of nights "downtown."

I've been to Cozumel twice and booked once on two weeks notice (April)
and once on one week (February).  Bay Adventures got me good deals both
times.  The trade offs are you can get a good deal on a hotel room, but
the airfare will take most or all of it back.  Either way, what will you
do if there's a hurricane heading towards Cozumel on Sept 17?  Can you
cancel your vacation?  Ask about trip insurance if your are worried, but
as Gordon says the chances of any area getting hit are small.