Nickglover,
You mentioned that you're bringing your own snorkel gear - I just wanted to mention that it's possible to rent just fins on Castaway Cay, even though that's not specified on the information sheet. We brought just snorkels and goggles, since the fins were too bulky to justify purchasing and schlepping all the way from home. We hoped to rent fins on the island, but Guest Services onboard the Magic said we wouldn't be able to, only the total package was available.
I'm glad we checked with the actual rental place on the island, because (of course) we were able to get just the fins for $9/pair, and we borrowed the required vest for free.
The snorkeling is pretty good - we saw a sting ray as well as lots of fish, and my DH had a great time exploring all the starfish at the adult beach. But the snorkeling on St. John was amazing!
Have a great cruise!
April