Champagne reef is something you could do on your own. We just got off the ship and hired a taxi/tour guide to take us there (you negotiate the price for the travel to Champagne reef) They have a place you can rent snorkel equipment from there and you can even tip a snorkel guide to show you around (although its not really needed). This is a beach access and you are snorkeling within 20 yards of the beach. You can have a life vest on if you wish. It might be easier to just book something through the cruiseline. Unless you want to see other sights, just booking a tour guide to take you to the reef alone might be more expensive than booking a tour there. AND if you book a tour through the ship, then you know how much you are going to spend before getting there. It been a couple of years since we have gone so I can't give you a price.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013