The Pacific Ocean isn't too far away. How hard is it to build desalination plants to increase supply? How long would it take, and how expensive would it be, t

The Pacific Ocean isn't too far away. How hard is it to build desalination plants to increase supply? How long would it take, and how expensive would it be, to increase the water supply so there's no shortfall and some reasonable safety margin? Would desalination capacity built in Pacific coastal cities like LA or SF be able to alleviate shortages with all the new water being used locally? Or would you actually have to pipe the water away from the ocean to inland desert areas like Las Vegas and Phoenix? Are there pipelines for this already, or would they have to be built?