You may want to check out Capt. Marvin for snorkeling with the Sting Rays, you can pre-book or take a chance when you arrive, but there is that chance that its filled up for the day, here is the link:
You may also want to read through this site and see if there is any other excursions you may want to do:
In looking at excursions, you may find some that the cruiseline offers that you can't find anywhere else, but some of them its true, you can do them cheaper on your own, so compare what the cruiseline offers and go from there. Just remember, some of the more popular ones do fill up, so best to book as soon as you are onboard.