From where to where? I am in the Northeast and we have gobs of water. But it would be insane to try to move it further west.
Desalinization seems to be the most effective and efficient method to date. But getting factories made on SO-CAL beaches seems to be a non starter.
Maybe they can build them in Mexico and bring the water north in tankers.
Cheaper solution - make each illegal bring a bucket of water when they cross the border.