Coco Cay is a private island. It's a great beach day. The ship's crew mans the stations and does a bar-b-que. There are water toys, snorkeling, chairs, umbrellas, bars, etc. We've done many cruises in December and have had both good and bad seas. Two years ago on the Royal we had horrible weather for the first 3 days of our 14 day cruise. We had 14-20' seas, rain and lots of wind. Last year on the Solstice, we had perfect weather. The day after we got back, it was freezing. It all depends on the fronts that are blowing through.
Have a wonderful time!