I use water from a water cooler that we have in our kitchen. It is filtered drinking water, and it has a hot and cold spout, so I use a little hot, and then the rest cold to get the water at the right temp.

I use the drinking water for two reasons. 1) just to make sure there are no contaminents, and 2) because we have hard water, and this way I don't get hard water stains in the bowls or in the tub if the water is spilled.