Cash is king. Small entry fees are required and cards are not accepted.
Invest in a snorkel and mask. Most places will have equipment rentals but it is nice to have your own.
Having an underwater camera or case makes all the difference. They are invaluable and worth the initial investment.
Biodegradable sunscreen and eco-friendly bug spray are a requirement at most cenotes. This is to preserve the natural environment and ecosystem.
Water shoes, although they are not chic, are practical. If you can stomach wearing them it will give you the ability to jump off cliffs, climb on rocks, and keep your pedicure perfect.
Go early or late because mid-day is peak time and will be busy.
Two cenotes to miss:
Zazil-Ha is located in the vicinity of Tulum. I found it to be over developed and way too crowded.
Cristal is also located a short drive from Tulum. It is included in the price of Escondido's entry (mentioned above) and located directly across the street. The water was stagnant and filled with algae. If I had known this prior to purchasing our tickets I would have requested Escondido only.