When you say 18 year old scouts, I take it you mean Girl Scouts of America? Anyway, all of the ports that you mentioned are great for snorkelling. In Grand Cayman, there is a great snorkelling excursion and you can swim with the stingtrays. Cozumel, there is snorkelling and lots of shopping. Belize is a snorkelling dream. Perhaps you should ask the girls what they would like to do. You can access some of the shore excursion information and descriptions (but not prices) on the Carnival site. This might give them and you a better idea.
I think if you discuss things before hand, it will be better than waiting until you get on-board and then deciding, especially with 18-year olds. Have fun!