We did Snuba in Key West and in Grand Cayman this past week (through Celebrity, but I know DCL uses the same company in Key West... don't know about Grand Cayman). It was really great... on both trips there were a couple people who just couldn't get comfortable with it and didn't really go down. But my husband and I are very good swimmers, very comfortable with snorkeling, and we had each breathed on regulators before (once a Scuba resort course and once the behind-the-scenes thing you can do at Living Seas). So for us, it was easy to get used to the equipment and enjoy ourselves.
The Key West one was a bit better... on that one, we took a long boat ride out to a coral reef. Tons of colorful fish and beautiful scenery.
In Grand Cayman, we walked from the pier to a dive shop and snuba-ed right off the beach behind the shop. There is part of a shipwreck down there and also a bit of a reef. We saw fewer fish but more HUGE fish who live really deep down... so it was interesting.
It's nice because you follow around a guide underwater who points things out to you. And there's an underwater photographer selling way, way overpriced pictures. (well, at least the Key West ones were... don't know how much the Grand Cayman ones cost, bc we needed to get back to the ship in a hurry)