I can't add much about thermal protection that hasn't already been said, but will offer a suggestion of a different sort. If your last dive day is in the cenotes, it is a great way to get all the salt rinsed off of your gear before travelling home
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djanni, how you react to the cenote dives is very personal. Personally, I have done precisely one ocean dive down there in the last six years and almost a dozen trips, because the reefs, even in Coz, can't hold a candle to the caves -- to ME.
When our group came down and dove Coz for nine days, we did two days of cenote tours, went back to reef diving for a couple of days, and then all looked at one another and said, "To heck with the reefs, we want more CAVE!" and did more cenote tours.
It's very personal.
On my first visit, we did the cenote tours that anyone can do -- Taj Maha, Ponderosa, Dos Ojos, and Carwash. That brief exposure was enough for four of the five of us to go on to get cave training and become Full Cave divers.
I've made 11 trips to the Yucatan so far, and plan many more. I have dived in more than a dozen different systems, and hardly touched what is there. My average dive nowadays is between two and two and a half hours, and a couple thousand feet of penetration. I have dived some systems multiple, multiple times (Sak Aktun, for example) and have yet to get bored. Every time I dive a passage, I see more . . . and when I don't want to go back where I've been, there is passage I haven't dived yet.
I utterly love the caves -- love the challenge, love the clarity of the water, love the formations, love the mystery of "what's around that corner?". If you offered me a choice between a week of reef diving off Cozumel and a week of cave diving, I would not hesitate one moment.