Welcome Aboard, Laurie!
I am so glad you navigated your way to the Disney Parks Moms Panel to ask your question about snorkeling! You are absolutely welcome to bring your own snorkel equipment on your cruise and use it on Castaway Cay in the 22 acre
Snorkel Lagoon off of the Castaway Family Beach. Disney Cruise Line does ask that you wear one of the safety vests that are provided free of charge at Gils' Fins and Boats or Flippers n Floats.
As you enter the lagoon, it is quite a swim out to the actual snorkeling area, so be prepared for that. I typically only recommend that strong swimmers even attempt to go out to the Snorkel Lagoon. The buoys floating in the water mark where you will find some underwater "treasures". There are also two lifeguard stands out in the lagoon, whom you can ask questions about finding the different underwater items.
As far as snorkeling at other Caribbean ports of call, you will have to check the minimum ages for the excursions or venture out on your own to a beach.
Please also be aware that snorkeling equipment is not allowed in the ship's pools.
I hope this helps with your cruise planning! Have a wonderful cruise in just two short months!
Happy Cruising, Tracy V.