I have used the water shoes while snorkeling at Coco Cay and they work well. I bought mine at Wal Mart for about $10, I think. They're not the best quality and the sole insert keeps coming loose, but they do the job.
I wouldn't bother with doing the snorkeling through RCI though. Toting those fins everywhere is useless, IMO. You can buy a decent mask and snorkel for around $20 and take them with you. This way, you know who's mouth has been on the snorkel last.