My wife and I have dived the cenotes in the Yucatan for the past 11 years, and while each one has it's own individual attraction, Casa Cenote has to be our favorite, for just what you are asking. You hear about the interface between the ocean environment and the fresh water of the cenotes, but nowhere is it better represented than Casa Cenote. On our last visit there, our guide took us up the right (North?) side, into any number of chutes and and channels that had nice populations of juvenile and adolescent salt water fishes. Our favorite is the blue crabs that populate the middle of the channel, who hide among the algae and leaves that have fallen to the bottom. Then we came down the left (South) side, and got into the Mangrove roots (they are actually on both sides) to see the environment they represent.
If you have only snorkeled Casa Cenote, you missed the extraordinary experience that it represents. Our last dive ended with a dip down to the base on the East end, at the exit to the ocean (there's a sign there), where you have an opportunity to clearly appreciate where the mixing is starting to occur, and the relationship that the cenotes have with the ocean and the fresh water underground rivers.
If you've been to Casa Cenote on snorkel and loved it, doing it on Scuba will blow you away.
