The food isn't very good (except for the desserts, which are quite tasty, and some of the smoothies), and perhaps some things need repairs and a bit of work, but otherwise, I really have no complaints about
Castaway Cay. It's meant to be a relaxing beach day with optional non-relaxing activities like parasailing, snorkeling, and bicycling for people who want to do those things. Why would you want to ruin that with a giant waterpark that has a ridiculous upcharge like Coco Cay? No, thanks.
For those wanting a pool, try swimming in the ship's pools while it's docked at Castaway. There won't be many people there!