Wife and I were there in May this year,and booked with Carnival since we were not familiar with any private tour companies and we wanted the absolute guarantee of getting back to the ship on time. Private tour companies might offer the same tour for a little less, but I think the tour will be shorter and no guarantee - stick with the ship tour if this is your first time. Now, we truly enjoyed our excursion. We got off the ship and were promptly lined up and boarded a nice air conditioned bus. Our tour guide was of Mayan descent and gave us a very informative briefing during our 30 minute bus ride. On arrival, they gave us a bottle of water and we divided into 2 smaller groups to the ruins. Wear good walking shoes and apply sun screen as it is hot and the tour requires walking on dirt trails, but is is easy if you can walk without any restrictions. These riuns are small, but still very interesting. And, the bus ride is a lot shorter than the more inland ruins which will take the entire day. We thought it was an excellent tour and the price was worth it. Go, and have fun...and take lots of pictures! We were back to the ship by about 1:15, had lunch on the Lido, and still had time to go back to do some shopping at the pier shops. Google Dzibilchaltun or go to www.locogringo.com/yucatan/dzibilchaltun.html
for a direct link to read all about it. Hope you have a great cruise!