This extraordinary prehistoric lagoon is fed by six thermal springs with temperatures ranging between an appealing 27°C and 30°C (80.6°F and 86°F). Its crystal-clear waters mean snorkelers and divers can view beds of water lilies, an ancient petrified forest and several fish species. Hundreds of families flock here on weekends, when it can get very busy. You can hire snorkeling gear from one of the many stalls inside the area (M$20).
Diving courses are available. The most highly recommended is Escuela de Buceo Media Luna, run by master scuba diver and oceanographer Ossiel Martinez.