I thought I'd post here after the really enjoyable trip we took out to this cenote a couple of days ago (and I mean Dzitnup/Xkeken). Hopefully it will help anyone else who want to go but is relying on public transportation...but I will also put a longer version in the "Attractions" section.
Easy to do using collectivos/taxis. We paid 20p each for the 10-15 minute ride, were let off right at the entrance. 61p each for admission now. Hardly anyone there around noon - in fact except for the attendant, Benjamin, we were alone for almost all of the 2 hours we spent there. Excellent facilities, food available, any equipment you might want to rent (mask, vest, etc.) By far the prettiest cenote we've ever visited (Good enough for the National Geographic cover), and the only time so far in our lives we've had tiny fish nibbling at our feet, screeching bats far overhead, and the sun streaming through a hole in the cavernous roof. 70p taxi ride back to town. Great day! Special thanks to this forum's own Wm Lawson, whose blog on this topic we read beforehand.