I wouldn't recommend it for castaway. When we took our trip Castaway was one of those ports that we were able to just sit back and enjoy. No rushing to meet reservation times, no hustling around. It was one of the most relaxing and pleasurable days of the trip.
The snorkeling you can pretty much just walk up and get your gear. There's plenty of space on the beach, and even free swimming stingrays in the water (which eliminates the need to reserve a spot to pet them)!
I wouldn't worry too much for planning on Castaway.