A water manager for United Water Conservation District named Steve Bachman explains succinctly why desalination is not the answer, in a story I wrote in today's Ventura County Star (here):
In a phone conversation, he explained further. Water districts can buy water from Metropolitan -- when it is available -- for $600 an acre-foot. "Desal" today costs in the range of $1100.
So there you go. Of course we haven't mentioned the huge carbon footprint, the infrastructure problems, the salts disposal...but do we need to?