GO TO COZUMEL!!! We were there at the beginning of the month and had a fabulous experience. We stayed at the Hotel Cozumel. It is completely up and running- including the dock. We dove with some people who were staying at the Casa Mexicana. They said that it was great too. The dives were all great. We dove Baracuda, San Juan, Columbia, etc. The small patchy reef off of Papa Hogs was severely damaged but the rubble has created very interesting diving, as fish, eels etc have adapted to new 'homes'.
Remember, the dive operators and Dive Masters that we depend on to take us on great and safe dives need the diving community to come spend their money and get the word out that the diving is still great! Plus, with fewer cruise ships in port, you have much less traffic at the dive sites.
All of the great restaurants (Primas, Palmas, Guido's, Pancho's Back Yard, and Rockin Java's etc) downtown are up and running. Chaudrai's grocery store is back to normal too. Our only real disappointment was that McDonalds and the Burger King next to the ferry dock were closed. If you have kids that need their french fries and chicken tenders, you can go to the Burger King behind the Chaudrai's.
(We took our 5 and 9 year olds)
All that being said, be sure to check on your specific hotel to make sure that it is at 100%. Many of those outside of the downtown area were completely devastated and won't be back anytime soon. Also remember that those areas that were damaged but are now open are often nicer and newer than they were before.