Sandman, you can snorkel the reef around the Blue Lagoon. There are boats and captains always at the wharf that can take you out or you can dive/swim from the wharf.

Other options are from Monkey Island, Reach Falls and I hear nice snorkeling at San San Beach.

I would suggest visiting San San Beach for the afternoon and asking them about active spots. This is a paid beach I believe $5 but worth it. They pamper their guests like the staff at Frenchman's Cove.

Oh yeah, I just remembered there are boats at Winnefred Beach and I see them take folks out. Maybe one day while there you can ask them.

I hope the above helps.