In addition to what bpd528 said, if you have your own snorkeling gear, you don't have to pay for the float vest. Just grab one at the shack where they rent the gear.
I highly recommend bringing your own gear. You can buy a cheap set for about what it costs to rent on Castaway Cay, and you'll have it for other ports and for future trips as well.
There isn't generally a lot of sea life when compared to snorkeling "hot spots." Go early, as people tend to kick up a lot of sand as the day wears on, and visibility suffers. Hubby has some photos he took while snorkeling...they are on my website at: www.castawayclub.com
Often there is a lot of sealife around the plane wreckage near the adult beach too, but last time hubby couldn't locate it. We have photos of that as well.
Barb
Visit the Platinum Castaway Club at: www.castawayclub.com
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Not worth $70 if you are a diver. Buy the gear at a discount dive shop and snorkle at St Maarten, St John & Castaway Cay on your own!
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