I haven't read much on this, but I have been trying it, just because I've noticed the different ways my body reacts to warm vs cool water. The trouble is that I don't like standing up the whole time; crouching makes me very aware of heat being lost through my knees; and then I wind up wasting a lot of time and water with the heat turned up.
The obvious solution seems to be wrapping my knees in something, which I've done a few times with success; it's just hard to form into a habit.
So far, my experience points toward cooler water being beneficial, not that my anecdotes are anything resembling scientific (even as self experimentation goes).
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.