Desalination will require nuclear power to be economical.
So... you want to deal with the anti nuclear lobby? Because I assure you... they're about as much fun as a bag full of leeches.
As to water conservation, Californian agriculture has not radically increased their water usage in decades and yet we have less water now then we did then.
That questions whether we have less water now then we did then or if there is another user of water that has increased their water usage.
Its not tough to figure out. The cities grew quickly and the water infrustructure was not expanded to compensate. So now they're drinking the farmer's water.
That what is happening. In all fairness, the cities should build the desalination plants since they're the ones that expanded their usage. What is more, many of those farmers have water rights that go back over 100 years. You and invalidate those water rights by eminent domain... but that's just a fancy word for legal theft. Yes, they compensate people usually when they do that, but often the compensation falls far short of the value.
In any case, your idea will generally mean less production in Californian agriculture. Conversation typically has that effect. Or if you prefer simply higher costs. If we shift to green house growing for example that will increase costs radically.
My point was that we need produce to be cheap. Conservation makes it more expensive. So my example was not bad... it simply touched on an issue you are unwilling to be flexible upon. Which is too bad. It means you're ultimately in favor of making produce too expensive for poor people to buy. Which is sort of fucked up... but I do appreciate that isn't your intention. Its just the next effect of not paying attention to the consequences of policies.
When all is said and done, please look at the final result. Then tell me if that's desirable or not... and then see if there was something up the chain that could have been adjusted to improve the situation. Just my two cents.