The water at Castaway Cay in winter is a cool 75 degrees, which is perfectly fine to swim in. If you only like to swim in warm water (like many Floridians), you won't like it, but it isn't freezing water or anything like that. In my hometown we have a cold water natural spring pool that is always 68 degrees and it's extremely popular year-round. Once you immerse yourself, your body adjusts and you can enjoy going in and out of the water at Pelican Plunge without being cold. The only time that wouldn't be easy would be if the air temperatures were chilly, but that rarely happens on Castaway, even in winter. It isn't good water for wading in just up to your knees without first immersing yourself, though. So if you like to hang out at the water's edge and wade, it's better to do that in the summer at Castaway.