The difficult part to do in secret would be physically transfering the water.
I imagine an anime, where thirsty Californian citizens pay bitcoins on darknet for water. Then they put on cow costumes (to maintain secrecy), go to farms, authenticate themselves electronically to an electronic cow-guarding system, drink water from the trough, and return home. The farmers pretend they see nothing unusual, just regular cows coming and going, to keep plausible deniability (I said it's supposed to be an anime).
Alternative idea: The farmers should produce and sell genetically modified animals or plants consisting of 99% or more water. For example, water elementals.
Yet another idea: The farmers should sell "tours on the farm" where people can go to see the farm. A visitor is allowed to drink 1 liter of water during the tour. Of course everyone would know people just come to drink the water and leave... but this would give everyone plausible deniability. There is no water being sold, officially.
I think we need a discussion thread for the californian drought going on. I would like to compile information in the main post and would like help compiling it. If we really are proud to be effective altruists then this is an area we should really figure out.
Any one have any good ideas on how we can help?