If you are talking about snorkeling at CC, I believe the bests are available free of charge.
If I am remembering correctly, the vests are more for visual reference for the lifeguards (so they can see you). They are like a bib that you inflate manually by blowing into them. So if you don't want to inflate them, you don't have to. DS had on one of the life vests that they provide to keep him floating and the snorkling vest over the top of it.
I didn't mind wearing it inflated because the swim was quite a ways out to find the hidden treasures and while the vest doesn't necessarily make you float, it does make it easier to float while checking out the underwater items.
Thank you ranidayz and jetskigirl, you saved me some room in my luggage and from a fight with hubby if I tried to pack a ski vest all the way to Castaway Cay!
Wait, there's lifeguards at Castaway Cay? Do they arrive on the ship too or where do they come from?