California should raise the price of water
This would do more than any set of regulations ever could. For example, the governor is not going to force people to replace their old toilets with newer, more water-efficient ones. But a higher price of water would encourage people to do that. A higher price would also give farmers the right incentive to grow the most water-efficient crops. It would induce entrepreneurs to come up with new water-saving technologies. And so on.
Some may worry about the distributional effects of a higher price of a necessity. But the revenue from a higher price could be rebated to consumers on a lump-sum basis, making the whole system progressive. We would end up with more efficiency and more equality.