I went thru the calculations a long time ago on how much energy it took to desalinate sea water. It was the equivalent of the water falling several hundred feet. A very energy intensive enterprise. There are some attendant equipment problems which add to the complexity and cost too.
The point being, that if fresh water is available somewhere it makes sense to move it quite some distance, even with the infrastructure costs involved, to avoid desalination. The preceding post I think has it right. Water redistribution makes much more sense (usually) than desalination.
This is going to be inexpensive energy. But It’s still not free.
This doesn’t have to mean more dams or redirection of entire rivers. I could see using a tiny fraction of the Mississippi for irrigation without disrupting ecological systems. That river transports a lot of water! I grew up fairly close to the Ohio river and I thought it was big!