Then all I need to do is desalinate drinking water.
“Distributed” home desalination for drinking water seems like the best approach in my mind, then people can pay as much or as little they need, but I have no real data to back this up.
On top of that all the brine that people produce in their homes would have to be disposed of, and I'm sure many people would just end up flushing it down the drain. So the water treatment plant would have to deal with highly concentrated, contaminated saltwater.
Sewage in particular will create hard deposits in plumbing that needs to be dealt with every few years at a minimum.
Frankly, unless you are in a rather extreme environment, like a desert, or a boat where you have to carry or make all your own fresh water, saving a few gallons on showering and washing is pretty inefficient. You could have a far larger impact by changing habits, and ensuring low flow appliances.
You at least need special soap: https://en.wikipedia.org/wiki/Saltwater_soap