Don't be afraid of the String Rays: They are totaly harmless. They looked and felt like a giant Portabela mushroom, like the guy in the R.C. commercial on TV said. Actually, they feel even softer. Like a baby's bottom the size of a big pillow swimming by under water. They provide you with little sardine/anchiovy sized fish to feed them. You would hold the fish between your fingers and place your hand underwater. Eventually one of them will swim over your hand and suck up the fish.
When I was in Grand Cayman I also did the tour of the Cayman Turtle Farm and Hell. The Turtle Farm was OK...not great, but OK. Especially if you think little turtles are cute. Just be careful when you pick them up. If they start to panic and flap there flippers in a frenzy, don't get them too close to you. They are surprisingly strong and will really slap you silly.
I was not impressed with Hell. It is basically a tourist trap. It consists of red wooden shack gift shop in front, and the eroded coral limestone rock formation in back. The grey rock formations in back concievably resemble the flames of Hell. However, I grew up in Miami. So, looking at coral limestone rock does not thrill me at all.(dig just 3" below the ground in Miami and that's what you'll see).
Make sure to get some Tortuga Rum and Rum Cakes while you are there.
So, that's all I can think of at the moment. Have a good time on your cruise!