I am assuming you mean this excursion: ST32: Kayak, hike and snorkel of Cas Cay
We did this in early Dec 2019 and had a great time. Make sure you have tons of sunblock as there is not much shade. The only issue we had was that DD (9 at the time) was not into snorkeling and it was very tough for me to head out and enjoy some snorkeling while leaving her to look at shells with others in the group that chose not to snorkel. It was my first snorkeling experience and saw some cool creatures. I think the neatest thing we saw was a small octopus in the shallows during the hike portion.
It is hard to describe how strenuous the excursion was. It required some effort, but not a crazy amount. We did not preorder any of the food prior to heading out on the water, although it looked pretty tasty. I think we went back the ship and just managed to slip into lunch at Royal Palace and then walked back out to do the Skyride up to Paradise Point.
Enjoy your excursion!