Used to do a great deal of scuba diving during my formative years... used to teach a class or two to boot. I always preached "fins or float" when you swim in mother ocean... Dave is most right, fins make getting around a heck of a lot easier but, perhaps more important... they provide an incredible safety benefit, pretty much for the same reason... should you encounter a current or any serious wave action they give you the edge you will need to power yourself out of harm's way...
Since Castaway is pretty much a man made and extremely protected and safe snorkling environment I doubt that you will encounter any of the above but foms still make a great deal of sense... you will also be required to wear a vest (float)...
Back to the basic premise of all of this rambling... anytime you swim in the ocean wear fins or carry a float... encounter a rip tide or some serious tidal surge and will be glad you have one or the other...