California does not consume most of what it produces agriculturally. Most of it is exported to the rest of the United States and beyond. If California were only producing enough food for its own residents, there would be no water shortage. Of course, you'd probably have no lettuce in your grocery store then (unless you live in California).

The point is that California is not overpopulated by the "enough water for its people" metric. Most of the water use in California is to grow crops that are sent out of state.