the poison?

The water was flowing from out of the rocks and was picking up heavy metals and toxic minerals. Even if that were the case I'd think that they'd at least take a quick bath to cool off.

If nothing else splash it on your face and skin to cool down your body...

A lot of desert water is poisonous. Deserts are almost always dried out places that were once water covered. Yes, that's a fact. And when you dry out something like a lake, all the crap that was in that lake is concentrated into toxic crap. Run water through that and you can use it to strip varnish off a boat hull. Alkaline water will eat holes through you like drain cleaner.