Well - good in theory - really expensive in practice. Every Desalination process I’m aware of consumes HUGE amounts of energy. California is NOT blessed with abundant, cheap power. Rather we have barely enough power to get through our hot summers and it’s some of the most expensive energy in the country. (Yeah - I know a lot of these expenses are due to poor planning AND shooting ourselves in the foot via heavy regulation.) However - that environment is CA’s reality.
So desalinization doesn’t work. Further - it’s impractical to supply the largest single user of water in CA with desalinized water. The largest user is agriculture - who uses something like 80-90% of the water available in CA. The Ag use is enormous. Again - not practical because of cost and just the vast amount required.
Building nuclear power plants with desalination plants colocated solves the energy issue. Waste steam is used to run the desalination process. Fresh water and sea salt are then just byproducts of the nuke plant.