There are plenty of hot places with enough fresh water supplies that trying to conserve water is of dubious value.

The problem with fresh water isn’t the overall supply of fresh water. It’s getting it to communities that need it, when they need it, and distributing it without contaminating it.

I think the biggest costs of sewage treatment is removing solids and hazardous contaminants. Diluting wastewater with moderate volumes of potable water may not mean much to the whole-cycle.

It's not nearly as bad as watering a lawn, where water goes out clean and comes back as contaminated runoff that has to be captured and treated or dumped.