Why doesn't the water surface of Liangshan hero Shui Bo freeze in winter?

My home is at the foot of Liangshan Mountain. Although the TV series is a bit exaggerated, it is not completely impossible. Because the weather is not particularly cold, or even if it doesn't last long, it won't freeze on the water. If there is cold air suddenly at this time, it is likely to snow and not freeze. This kind of thing can happen in our place, what's more, the water margin is located in Wuxi, which is warmer and easier to happen. Ten thousand steps back, there are such mistakes in TV drama skills, and there is coca-cola bottle in Water Margin! The director needs a scene of heavy snow. It is very rare for a place like Wuxi to snow heavily, so we can't expect it to freeze. The director will not go to the colder north for these big fanfare.