I grew up in corn snake country. In the winter, sure, stream water will get near freezing, but the snakes are all underground then. I could always swim in the water of piedmont to coastal NC when it was snake season -it wasn't nearly that cold. But like I said, it's probably not going to matter, the odds of the snake going right to the bowl and drinking in the first 10 minutes you put fresh water in there is unlikely. It's just an unnecessary step, that probably wouldn't be appreciated if they had to drink it that cold. Fortunately, in a matter of minutes, a small container will reach room temp.
It's a sweet gesture, but these aren't mammals that control their body temperature, and thus can appreciate a cool drink.