You probably can swim in December. The water is heated and unless the weather is unusually cold it will be fine.
Activity wise they have a ton of things every day in the summer when I have been there. I'm sure it's similar in December. Things like candy making, scavenger hunts,zumba, mojito making, fishing contest. There is aslo evening events like karaoke, Polynesian night and dueling pianos. Once a week they have a mashmallow roast too.