Note: said society is preindustrial and has ample access to flowing water, but not fossil fuels or radioactive deposits. It also has access to mountain hot springs, which I thought could also be used as “batteries” during times when streams freeze. This last point is what set him off.
He also really hates wind turbines.
The hot springs act as reservoirs, allowing the civilization to have a (reduced, of course) supply of liquid water during cold seasons. The water is passively heated by magma currents running underneath (akin to how hot springs generally form) so they don’t have to expend any additional energy to warm it.
It’s like a damn Jacuzzi. The civilization pretty much has on-demand hot baths.
Not sure if you were planning on using it as such (or how much you care) but hot spring water can’t be used for drinking water, at least not for any length of time.
It’s just for powering machines.