there are some really nice reefs just out of town towards the airport. it was some of the best off shore snorkleing i have done. i even managed to snag a short ride with a sea turtle (yes i know i'm bad but i just couldn't resist )
I snorkled quite a bit right off Playa Las Ballenas, right off the beach road down from Hotel Colibri. Some nice coral formations with sea fans, brain coral and colorful fish.