Welcome aboard, Toni! Congratulations on your upcoming cruise with Disney! All of my family's Disney voyages have been special, but that very first one was the most magical of all because everything was new and a surprise. I can't wait for you and your family to experience all of the magic!
Castaway Cay is one of my favorite places in the world. Disney's private island is only accessible for Disney Cruise Line guests, so it's almost like an extension of the ship itself with more activities, events, and lunch all included. Plus, there are extra opportunities for things like Port Adventures, and it sounds like you're already considering
renting snorkel gear to enjoy the Snorkel Lagoon. I love snorkeling there. Disney has hidden some surprises for you to spot beneath the waves, so if you're on the lookout for hidden Mickey's, keep your eyes peeled! The last time we snorkeled there, my son and I had the awesome experience of a huge sea turtle swimming right beneath us. Wow!
We've always felt super comfortable leaving a small bag with personal belongings on our beach chair, maybe covered with a towel, while on Castaway Cay. But, more often, we bring our phones with us in a water pouch on a lanyard so we can take pictures while snorkeling. Check those out online. I ordered a set of four of them for maybe $20 online, so do a search and you might find something similar! But, I don't suggest bringing anything of great value off the ship. If you do want to leave valuables on the shore, and don't feel comfortable simply tucking them in a bag under your chair, you may consider one of those beach chair lock boxes I've seen advertised. Just do an online search and you'll see a wide variety of those.
I hope that helps as you get ready to set sail, Toni!
Marcy