Castaway Cay

If you ever get to opportunity to go on a Disney Cruise in the Caribbean odds are you will make a stop at the private island known as Castaway Cay. On the island is several age specific areas as well as lovely family beaches with snorkeling and underwater treasures to be found whether on your own or with a tour. When docked you can see the Flying Dutchman, Davy Jones' ship from Pirates of the Caribbean, and once you are on the way to the beaches there are many photo opportunities with it as a background. From the cruise ship you walk a little ways and than can catch a tram down to the beaches.

Down by the beaches is where you can buy or rent all your beach necessities. There are also random character meetings all around the island, so if you spot Minni Mouse in her bathing suit don't worry you are hallucinating! The first beach areas you come to are the family beaches and are fairly nice but don't seem to have much in the way of shade and tend to have more people at them all through the day. Walking down farther you might pass the younger children play areas, Spring-a-leak, Pelican Plunge, and Scuttle's Cove.
The teen designated area was still being built when I last visited in October 2010 and was not very fun. There was very little shade and not much to do but they were still working on it. Farthest from the ship is Serenity Bay the adults only beach. Sadly at the time of my cruise I was 16 and not supposed to be down there.
I personally am afraid of the ocean so I stuck to the beaches and hanging out with friends from the teen area but everyone I saw doing snorkeling looked as if they were having a great time and I know there are lots of neat things in the Snorkeling Lagoon.

Spend your time wisely if you are planning on doing more than lazing on the beach as there is lots to do on the island. If you are stuck on what to see you can always ask a cruise director or a crew member what they would suggest as well as check out some other Disney forums to get more opinions!
Some photos from my visit: