Willie's has been running tours out of Cahuita for well over a decade: the snorkelling trip onto the reef inside the national park, which you cannot legally do alone, along with the Bribri reserve, the sloth sanctuary, chocolate farms and the day over to Tortuguero.
The park reef is the local speciality. It is the largest living coral reef on the Costa Rican Caribbean, it is fragile, and the rule that you go with a certified guide exists for a reason.
Their site is built entirely in JavaScript and publishes no telephone number; the only contact it exposes is the WhatsApp widget, which is where the number above comes from.