Ahoy, Patrick!
We loved spending time in the 20-acre snorkeling lagoon on
Castaway Cay! It is quite a swim out to the underwater "treasures", but once you find them, it makes the effort totally worth it. Be sure to bring your flippers along with your masks and snorkels, they are definitely needed.
Passengers who bring their own snorkeling equipment can visit Gil's Fins and Boats or Flippers & Floats to get a complimentary yellow dive vest. These are required for all snorkelers to wear while they are snorkeling, whether in the lagoon or in the swimming area.
I highly recommend heading out to snorkel early in the day. Since the snorkeling area is out further than the swimming area off of the Castaway Family Beach, as the day progresses, more and more sand gets kicked up into the water, making the snorkeling area "cloudy".
I hope your family has a wonderful time on your cruise! Please visit me again here at the Disney Parks Moms Panel to ask more questions as they arise.
Happy Cruising, Tracy V.