It's amazing what you can do when you've run out of water... But you are right, it would/will be a massive undertaking. I wonder if desalination will become water's "fracking", something we only turn to due to high price of what we used before or unavailability of what we used before.
I don't think the price will be near what would be needed to make desalination profitable without subsidies, though. The state could choose to maintain current usage and pay whatever it takes to maintain it, but it'd be at a huge loss. The main user of California water is agriculture, and desalinating water just to use it to grow rice is a good way to burn money.
