Despite California's attempts to "crack down" on the ALS challenge and such, there are still companies sourcing bottled water from the state that should likely be dealt with first.
Agriculture is (obviously) the largest source of water consumption in California as well.
Beyond that, there are some wacky laws (and logistical issues) complicating the import of water from places like Canada, but in my opinion, the next step should be to curtail (or impose costs upon) the alfalfa hay being exported from California to China.