We did this on our first Disney Cruise in 2003 when DS was 9 and DD was 6. We've been on two DCL cruises since (Mexican Riviera and Med), and we still refer to this excursion as the most disappointing that we've taken. Perhaps it was an off day, or perhaps it has changed, so I'm curious to hear what someone who's taken it more recently thinks.
I think we did the Atlantis one in Grand Cayman - it was located just to the right of where the tender boats dock - we did this about a year ago. It was me and my younger son who was 5 and we really enjoyed it. It was a great way to see the reefs and the fish for him since he cannot snorkel and won't get his face in the water.
I would do this again - sort of like snorkeling for those who don't want to get wet!