I've wondered this too [hello fellow storm person]. The winter temperatures are around 3c. Of course, pools seem incredibly broken right now; like most things in the game, so I don't think it's possible to get a clear answer. I don't really care for pools so I'm just making a pool-less park for now. I will say my water coasters have no decrease in ridership outside of the queue bugs.
Additionally, I have a hat stand that doesn't sell any hats, and I don't know if that is related.