Where are you staying? I would call them about snorkeling
As for other boat excursions just go down to the beach where the Ferry comes in and ask one of the guys who has a boat. I forgot the guys name who took us whale watching but he was one of the guys who was shuttling people off the ferry in the morning. We simply waited when they were done unloading the ferry passengers and asked him. Knowing Spanish may help
You wont be there during whale season but most of the boats would be able to take you sightseeing or fishing. I don't know if any of them have snorkeling equipment though that's why I say maybe talk to hotel about it.