OK, but right now, the plants don’t exist. Also, the plants take energy to run. Unless it’s coming from clean sources (renewable/nuclear/hydro), the water coming from them is environmentally expensive. Note that California is too ‘fraidy-cat to build more, modern nuclear power plants.
Funny you should mention salt water showers — quite a few old hotels at the Jersey Shore had salt water taps in their tubs, since bathing in sea water was considered healthy.
But practically, not everyone wants salt drying on their skin, and I suspect that seawater would also be hell on valves and copper pipes.
