We dove in Dos Ojos last week. We talked to a lot of people in Mexico and had this op recommended to us.
http://www.gocenotes.com
A couple of notes about them. They were the ultimate in professionalism. This was our first cavern dive and we felt great about what we were doing BEFORE we got in the water. The feeling came from the thorough briefing we got - Safety was paramount to these guides. Also, and this is the most important, our first dive was 59 minutes (if you read other reviews - on average thats close to 15 minutes more than usual). Why - because our guide was in no hurry to get the job done. He was more interested in seeing that we had a comfortable dive and took the time to appreciate the beauty of where we were. We noticed other operators were in and out a LOT faster. Our guides also arrived at the cenotes a little bit later in the day, and when we left - we were the only ones left at the site. Why? Well, as they told us, they try to set it up that they are the only ones around. We had a great time with Eric and Luca - our guides for the day. I would HIGHLY recommend this outfit and would also recommend you ask for these guides to take you. You will get a better tour than most operators show you. Demonstrate great boyancy and you will be taken to some amazing places!!! Don't know how big your group is - but remember - maximum 4 divers per guide. We did (and you should too!!!) 2 Tanks - the first in the main part of Dos Ojos where it is more open and more natural light. The second tank in "the bat cave" - a lot tighter and a lot less natural light. A light lunch included between the two. As I said - demonstrate good boyancy and show the guide you are comfortable and he will show you a few things you would not otherwise normally see. Believe me when I say - the safety aspect of an operator is always THE most important thing I look at. I would trust Eric or Luca with my life - which when you think about it - unless you are cave certified - you are doing!
We came to this company through another outfit we were reef diving with at X-Caret. The instructers there recommended them - as well as the location. Dos Ojos Cenote is absolutely the site to dive for your first Cenote.
The above post is also correct - enter through the main entrance and swim to the bat cave. Climbing the ladder is a joke and completely unneccessary - don't even know why it's done.
If you have multiple days available - do ALL cenotes. Skip the reef stuff. You can dive reefs all over the world - the experience of the cenotes is unique and as definite must do. We did a week of reef and only the last day we decided to try the cenotes. We were very disappointed that we didn't do the cenotes earlier. Even though we had a VERY close encounter with some spotted Eagle Rays, a school of 5 Mantas and tons of turtles - the cenotes were THE highlight of the trip.
Also, our op brought a camera man with us. We have an AWESOME DVD memory of our dive that you too will be able to get for $70 ($50 for VHS)! Well worth it. Great production and great camera work. Bring it home - show it to your diving friends and watch the envy!!!! Nice thing about the camera guy - he almost never talked to us about the video. He was more interested in our enjoyment of the day itself. At the end - he told us if we wanted a DVD or Video, just let him know. Then he walked away. We had to go after him to tell him we wanted it - no presure at all. He delivered a professionally produced and editted DVD to our hotel the next day!