A new desalination plant is in operation in San Diego county. Another is being built in Orange County. Your point is well made, California needs to increase desalination plant capacity more than it needs bullet trains.
Democrats prefer to rule under conditions of resource scarcity rather than abundant resources as the former promotes government dependency and the latter promotes individual freedom.
Fifty-one cents per metric ton—a cubic meter—better than 261 gallons of fresh water as provided by the desalination plant. That’s the lifetime plant cost for that quantity of water for economics break-even. Cost comparison plants worldwide:
https://www.researchgate.net/publication/272756219_Desalination_Technologies_Hellenic_Experience