CA exports a large portion of its agricultural products, including several top exports that are very water intensive to grow (again, almonds, walnuts, dairy & beef).
Moreover, current irrigation methods in the valley are not ideal for water conservation. Many farmers have water rights dating to the 19th century allowing nearly unfettered access to whatever water they can find, providing no economic incentive to invest in conservation infrastructure.
There is no Californian Malthusian crisis, just a story of poor resource management.
I feel like I can also be sure the export isn't really going to drop much with people coming in, but the domestic usage will rise if you keep letting people in. So the mere fact that we export doesn't really seem to help things.
There is a set of assumptions in your argument that I believe are false.
1. Unregulated water use that allows for production of export cash crops will continue
This could easily be fixed by regulating water use properly, such that farmers have economic incentive to grow crops that are less water intensive given that CA is not water rich.
2. Market prices of current export and current import crops will hold constant regardless of growth
That's not how markets work.
If there was a sudden surge in lettuce demand, farmers in Salinas would respond to increasing prices by switching some fields currently used for export crops to lettuce, which would be consumed locally.
This is aided by point (1) above, where we could push this shift with water regulations.
On the whole -- your concerns seem best addressed by sensible regulation of water in CA, which is a long term political problem. Opposing the farm lobby is well known third rail in the CA Leg.
Nativist population control programs are not the solution to a market failure of water distribution, however they might be implemented. Opposing housing construction is even a really poor way to implement a nativist population control program.
