I just got back from a mostly diving trip in Cozumel/Carmen de Playa. I'm not as avid a diver as I am a sailor, cyclist or backpacking, and would never undertake a diving vacation except that my attempt to organize a sailing trip in the Caribbean completely fell through! Unlike diving, cycling, or backpacking, sailing requires a full crew to make full use of the boat (to split costs), and I'm also not so competent a sailor that I can sail a boat short-handed or single-handed.
Arturo told me about the new Virgin America direct flights to Cancun with the special promotional pricing, and given that he'd already done all the research necessary on the area for a previous trip, I went about organizing a dive trip. Matt Vera, Tracy Ng, and Zaheda Bhorat chose to join me on this outing. Matt and Tracy wanted to complete a dive certification course, while Zaheda wanted to relax.
We signed up for the 5-night/3 days package at Scuba Club Cozumel. At $100/night for 5 nights including food + three 2-tank boat dives, Scuba Club Cozumel is a fantastic deal. The food was wonderful as well, and I consider it money well spent. The diving was very similar to the rest of the Caribbean, so nothing to write home about, except for the Wreck of the Felipe Xicotencatl, which was absolutely the best dive I did at the dive club. As a relatively new wreck, you get to explore all the rooms inside, and experience what it's like to be a wreck diver, wich close quarters maneuvers. This is a technically challenging dive and well worth the effort.
The club is full of avid divers, the kind of people who own their own dive equipment and do 100+ dives a year, so there's not much emphasis on much except diving. The equipment rental is expensive, but since you'll be doing so much diving, things like dive computers are a necessity. Matt and Tracy rented dive computers from the Bay Area, but both of those broke, while the one I rented from the Scuba Club had no problem.
Then we moved to Carmen del Playa to do 2 dives in the cenotes with Pluto Dive. These guys were picked because they were the only folks who ever responded to e-mail in Carmen del Playa. The dives in the cenotes were amazing. You basically jump into a freshwater cave and swim down into the caverns. This is like spelunking but without the scrapes and bruises and squeezing around things. With neutral buoyancy you basically float through the caves shining your flash-light at stalactites and stalagmites, seeing the glorious insides of the caves. At one point, we emerged into an underground cave with just a few air-holes, and could see fossils of tree roots on the ceiling (as well as other fossils), and could see the roots of trees coming through. Unfortunately my camera flooded just before the wreck dive, so you'll have to wait for Tracy and Matt's photos. The water was also the clearest I had ever seen. The transition from freshwater into salt water has to be experienced to be believed! Up until the cenotes dive I didn't think I would return to Cancun, as the diving wasn't any better than what I had seen in the rest of the Caribbean, but having seen the cenotes dive, I could see myself coming for more. These two dives were definitely the best dives I had done. Given that round-trip flights to Cancun are currently $150 or so, I recommend you do something about it before Virgin America's big discounts are gone.
We visited Chichen Itza on our last full day. The long bus ride (5 hours!!) was no fun, and while the ruins were cool (hey, virgin sacrifices... ball games to the death, hearts that get cut out of enemies and then sacrificed to serpent gods --- you just can't make this stuff up!), I thought I could probably get as much out of reading the Wikipedia page. I guess I'm just not cut out for cultural explorations. Worth going to once, but I won't do it again.
Carmen del Playa's beaches are very touristy, and they are nice, but not as nice as what I saw in the Virgin Islands. I definitely think that the Virgin Islands is still the best of the Caribbean, and recommend a trip there instead, especially for those who are sailors. But all in all I had a good time, and will consider returning to do more cenotes diving.