Welcome aboard, Shiela! My kids are about your son's age and love sailing with Disney! If you're only looking to book a Port Adventure in one port, Nassau is the one. Castaway Cay offers some great adventures too, but there is so much already included in your day there, like waterslide fun at Pelican Plunge! The snorkeling lagoon and bike rentals are great fun, but there are plenty of rentals available that day so you can always choose to do those once you're on the beach without booking ahead.
Key West offers some great adventures too, but if you don't find one that appeals to the family, it's a great port to explore on your own! Your son will love the Pirate Museum, and though it's included as part of some of the Port Adventures,you can also just purchase your own admission in town to the Pirate Museum! My son loved that place! Then, head back to the ship early to ride the AquaDunk over and over with shorter lines while other guests are still in port!
In Nassau, guests will be able to debark the ship around 8:15 and the All Aboard Time is 5:15. I agree that
Atlantis Aquaventure is expensive, but you'll have all day there. There is a half day option too. Guests on
Discover Atlantis don't get the waterpark, but get to explore the grounds and lost city replica aquarium at the Atlantis Resort. But, honestly, an 11-year old boy is going to have more fun at Aquaventure than just exploring the aquarium at the resort, and Aquaventure includes the aquarium and the waterpark. By not booking Port Adventures in Key West or Castaway Cay, you could put that money toward an all day adventure at Aquaventure, and still have so much fun in the other ports!
I'm out of space, but stop back with more questions, Shiela! We're happy to help!
Marcy