GoCorcovado is a Drake Bay operator selling the full local set: Corcovado at San Pedrillo or Sirena, snorkelling and diving at Caño Island, dolphin and whale trips in season, mangroves, and a well-regarded night walk in the forest behind the village.
Caño Island is the one people are most surprised by — the visibility is good, and it is one of the better snorkelling sites in Costa Rica.