Get a map and a car and check them out instead of spending money on a guide.
I'm sorry, but this is TERRIBLE advice. Some of the cavern zones in MX are enormous and very confusing (I've had trouble figuring out where to go when I decided to take an alternate route out of Grand Cenote, myself -- in the daylight zone. I couldn't get from where I was to where the light was without going back into the darkness, and I know the place.) If you lose the line, you may find one that heads into cave by mistake. Please, anyone who is reading this, do not believe that because there is a good safety record for OW divers doing the tours with a qualified guide, it is fine for OW divers to do the same dives by themselves. It's not.
Cenote tours are a big business in Quintana Roo. There is a set of rules by which the guides are supposed to govern themselves, but adherence is voluntary. I have seen outfits breaking just about all of them, including diver to guide ratios, availability of lights, and not having the guide in cave gear. It's worth doing some homework before signing up with someone.
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!