Water actually has the highest volumetric sensible (no phase change!) heat capacity of any substance on earth.
We don't rely on water vapor -- the problem is that at low temperatures there's very little vaporization that can even take place. This limits the heat transfer. Though there are alternative approaches that use this effect more fully.
We don't rely on water vapor -- the problem is that at low temperatures there's very little vaporization that can even take place. This limits the heat transfer. Though there are alternative approaches that use this effect more fully.