I did that same cruise back in 2008 and loved it. DW and I love water activities so we have done snorkeling and stuff like that. However, something you must do in Grand Caymen is the Stingray city. Now if you have small children this may not be the best but I’ve been all over the caribbean and that place has the best experience with Stingrays.
The reason I love it so much is because every other sting ray experience in the caribbean they have them caged up in a lagoon and the barbs have been removed with makes it nice and safe for everyone. At stingray city they are live. They take you out to a sandbar off the coast and there is usually about 10 to 20 boats there and 100’s of black dots swimming all around the place. The water is about waist deep and the only basic instruction that they give you is “Watch were you are putting your feet you do not want to step on one”. The instructors pick them up and feed them. My favorite thing to do is use the snorkel and float on the water you can watch a ton to stingrays just glide right underneath you. It is awesome. My DW says that everytime we have gone they love to bump into her and looking for food and they doing mind you touching them. They have been so use to people since tourist are brought out there for years. But these are as real as they get, no cage, no barbs removed, just live and real.
I put up some pics from the last time I was there
Lastly, you will love castaway cay. I would take another small Disney cruise just to go back there. We relax in the water, snorkeled, went on Jet Ski (awesome), and took a bike ride. It is the best.