So splitting California into two or more states would go a long way to solving the issue?
That won't work because of water rights.
Why is this not a federal issue anyway? How many watersheds extend across state/national lines?
Northern CA sends a lot of its water to Southern CA. The Owens Valley is where the LA gets most of its water. There are water deals between different states. San Diego gets the majority of its water from the Colorado River. Seven states have agreements on how much water they can take from the Colorado River.