Blue Cenote is 5 miles north of AKUMAL, right off the highway.  There are two signs, the first is 'Blue Cenote 100 Meters'.  Park for free, pay the man 50 pesos each, walk down a stone path in the jungle, and you come to an oasis.  A Limstone Pool of fresh water, say 68 degrees full of snorkelers, and amazing fresh water african fish.  Catfish, Jack Dempseys, etc.  You can jump off a 15 foot cliff in the pool, snorkel, relax.  Some brave soul swam through some shallow caves, very dangerous.  However, one of the true highlights of the trip.  There are hundreds of cenotes in the area, this one is not over priced, and extremely convenient if you have a car.