Desal can work for residential use in cities. But desal for agriculture is supplied by mother nature and it's called "rain." Replacing agricultural rain with man-made desal would require a 10x-1000x scaleup in cost and energy and area over the desal plants now feasible. In most cases it would probably be cheaper to move the farms to rainier areas.
I still don't think I understand how that helps Midwestern and Southwestern states other than California, unless you mean the goal is to give California a way to stop using as much.