On the shore excursions - snorkling from Montego Bay was good. The raft excursion was fun but after we refused to buy a calabash from the "captain" he became very surly. Grand Cayman - Sting Ray excursion was great. Cozumel snorkling was a bust - would definitely do snorkling on your own there. We were taken by boat to a beach and then after all were outfitted were told to follow the flag. Everyone in a bunch bumping into each other; and I became very tired. Turned out -- nothing to see there, no one could figure out why were were following flag.
I think most tours would have been better if we would have done them ourselves. The Sting Ray one was very good, but much cheaper on shore. Carnival provides a list of beaches on cozemal to taxi to, and those people were happy.