I haven't been on a cruise before but have done the Snuba experience off a catamaran in Aruba. It was definitely a memorable and great experience but between all the people on the boat we didn't get a whole lot of time to use the Snuba attachments. There were four or six but with so many people I think we had a max of ten minutes each. The other thing to take note of is that if you don't know how to dive down, it's useless.
If you can do a Snuba experience off a pier, it's probably worthwhile since you would probably have much more time on the Snuba tanks.