When we sailed the Fantasy in January, the "Snorkel with the Stingrays at Blue Lagoon" excursion, from the ship, was $45.00 per person (adult and children same price). It was a 5-5 1/2 hour tour and your choice of departure times was 9am or 11:30 am.
We didn't do this tour, but talked to some folks that did and they seemed to think it was great.
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04