We did a sail/snorkel to Buck Island last year and there were snuba boats in the same area. They looked like they were having a ball. It looked like it was a lot easier than scuba, no tanks to contend with (even though I like to dive). I probably didn't help much, but they do go to normal dive/snorkel areas and the folks looked like they were having fun. Have a great cruise!
Some of my friends did the snuba thing. They had been very excited about it. But, in the end, it sounded like they didn't get much bang for the buck. Another friend did the snorkeling and liked it.