I just got back from the Jubilee's Western Caribbean Cruise - It was amazing - I would highly recommend the Snuba excursion. I had a couple of friends who loved the Mayan Ruins but the Snuba was absolutely fabulous. It also left alot of left over time for shopping and enjoying the downtown. Have fun!
I sailed on the Carnival Paradise on July 1st, and toook the excursion to Tulum - it cost $ 68, and I thoroughly enjoyed it. We had an excellent guide who was very knowledgeable about the site. He definitely sparked an interest in mayan history for me, and the first thing I did when i got home was to buy a book on the Yucatan Penninsula, and Mayan history.
However, it can get VERY hot as there is no shade. The tour gets back to Cozumel around 12-1PM which leaves a lot of time for exploring the island. If you are makinfg an extended stay at Cozumel (midnight) - be sure to go to Carlos and Charlies - definitely an enjoyable experience.