The balneario Las Pozas is a spring fed series of pools in a beautiful setting near the Rio Grande de Catemaco half way to San Andres Tuxtla from Catemaco. Usually deserted, the balneario swarms with local people on hot summer weekends.
Access is via the turnoff of highway 180 to Chilapan, (gas station) and then along a bumpy dirt trail towards the river. A sporadic restaurant served me some great fried chicken, and the waters were surprisingly cool.
Unless you intent to swim, the entrance fee of 30 pesos is usually waived.