I was able to snorkel 4 cenotes my last trip, 2 were just south of Tulum.
Crystal on the west side of the highway, basically a bowl shaped pond, contained Astynax tetras and live bearers, but no cichlids.

Escondido on the east side of the highway, larger and wilder, it had livebearers, tetras and xC uropthalmus.

Cristalino was just south of Puerto Aventuras, small, and a bit of a cavelike amphitheater. While snorkeling the sound of jungle birds was incredible, It was full of JDs, large sailfin mollies, catfish and I saw 1 large sleeper goby.

And Eden Cenote, bit larger, filled with JDs and mollies, and a few large xC uropthalmus.

Each cenote had a $5 entry fee, all but Eden were walking distance from the highway, each had a parking area, bathroom facilities were a bit rustic, and bringing your own TP is a good idea.
I only traveled during daylight, but at no time felt threatened.
There was a fabulous little restaurant just south of Tulum and on the beach called Zamas, it also had a quaint hotel attached.
