We were "rained out" of our Castaway Cay day last year - was it disappointing? Sure. But we made the most of it. All the characters that usually met on the island were relocated to the ship, and extra activities were planned on board (trivia, movie showings, plus more in the kids club that I don't remember because it didn't pertain to us). We figured if we went swimming in the ocean, we would get wet anyway, so we went out onto the island. The cruise staff provided us with towels and complimentary ponchos, as well as dry, warmed towels for us when we returned to the ship.
Yeah, the weather was pretty miserable, but we made the best of it. We swam, got a drink, poked around the shops and managed to eat at both the adult BBQ and Cookies Too. We only lasted about 3 hours before we returned to the Fantasy to play some board games and watch movies with room service. It's not what I had planned my CC day to be, but you work with what you get. IMO you pay for the ship when you cruise, so the more time I spent on the ship the more I was getting my money's worth!