If you check out the Excursions 11/25/07 thread, I just posted a detailed discription of the excursions we did the week of 9/16 several of those are the same that you are doing.
I did the turtles in Grand Cayman about 16 years ago. I didn't think it was that great. They take you to a turtle farm where they breed turtles (mostly for food)and you can pick some of them up and hold them for pictures. In my opinion it wasn't really worth the money