Hi,
The water parks are never super crowded in the winter months. First of all, the locals would never dream of going to a water park in December! And secondly, the weather may be too cold to swim, even for a robust Canadian like you!
Edited to add: I'll defer to FeralPeg's post below; she knows better than I do since she lives there! I had never seen a post like that before about BB or TL being full during the Holiday season.