Weather in December is changeable, but I will tell you that when we were at Yacht/Beach in December of 2000, it was chilly. Maybe 40's in the morning warming up to 60s in the afternoon. But did that keep us out of Stormalong Bay??? No way, no how! The water is pleasantly warm. If you don't mind the chill when you get out, you're in business!!! And we weren't the only ones - the place filled up in the afternoons. If it's too cold, there's always the hot tubs, which feel great on tired feet and legs.
We'll be there in December again and I'm definitely packing my bathing suit.
We were there on 10/14-10/18 and you didn't have to pay to rent the tubes. They were first come first serve, as they are renovating the area that used to house the tubes/lifejackets.