Another great DM for Cenote dives is Matt at Blue Angel. He was on the National Geographic expedition that mapped the caves under Tulum, and is a excellent cave diver and all-around instructor.Dennis, as mentioned, is a very good guy. Two other delightful people I know who do guiding are Natalie Gibb and Jason Renoux. Both are fun to be around, knowledgeable about the caves, and safety-conscious. Plus Jason has just got his certification to teach through Intro to Cave, so if you got the bug and wanted to do a cavern class, you could go ahead and do it with him!