Ahoy Deborah!
Happy Anniversary! What a wonderful way to celebrate - with a Disney Cruise Line vacation! You're in for a magical time with Captain Mickey and his crew.
Castaway Cay, Disney's own private island, is amazing - it's truly my happy place. Every time we visit, my husband talks about wanting to work on the island so he would never have to leave! Disney provides guests with complimentary beach chairs and umbrellas on the island, and you'll be able to pick up fresh beach towels as you head out to find your favorite spot to relax. There are no storage lockers for your belongings, but I've never felt uncomfortable leaving a beach bag with a few necessities (sunglasses, sunscreen, small camera, etc.) on my chair. I usually either cover my bag with a towel or place it under my chair when we head to the water. The chairs are quite close to the water, so you can keep an eye on your things while you're swimming. If you'd like to secure a spot closest to the water, I encourage you to be one of the first ones to debark when you reach the island.
There's no need to bring cash to Castaway Cay - all you'll need is your Key To The World card. Your KTTW card can be used to purchase any souvenirs or cocktails on the island. Lunch is provided and is included in the cost of your cruise! You'll find a fabulous island buffet awaits you at Cookie's BBQ or Cookie's Too. (Make sure to save room for some delicious soft-serve ice cream - it's all-you-care-to-enjoy!)
I hope this helps answer your question. Have a magical day! - Diane W.