This is silly.
California is full of incredibly bright people. This has been though of before (in face a large desalination plan went on line this year in San Diego and Santa Barbara brought its 1980s vintage plant out of mothballs this year). Others have been talked about.
But what exactly would you plan to do with all of that salt? And whatever other impurities might be pulled out? It's generally mixed back with some water and put back into the ocean which is not without its impact on wildlife and ocean ecosystems.
Desalination makes some sense in places like Israel or Saudi Arabia where water is quite scarce, and California will probably have to use it more in the future, but implying that Californians are dense and stupid because they haven't considered this is insulting and just plain wrong. It's complicated and very expensive to run destination plants. Of course if the energy is produced by solar in some way that reduces the environmental impact, but there is no such thing as a free lunch.
British people don't always have all of the best engineering ideas in the world. Trust me, I've owned a British car (a 1969 Morris Mini Minor).