Referendums can do almost anything in California can't they? Pass a water reform law as an amendment to their constitution that states the judiciary has no power to interpret the amendment. Declare war on the pacific and declare pi to be 3.

Arent water rights mostly a federal issue at this point?

No.

The water compacts between states are the responsibility of the federal government, but water rights (i.e. landowners' right to access ground water and sometimes river water) are state matters.

Why would the federal government have any control over a states water rights? Except on water on federal land the US government has no authority to control a state’s use of its water, nor should it.

Climate change === mass migration.

Right now all the people out west think that is something that applies to people in third world countries. In the US it's going to be an attempted migration of the west's middle class to other regions.

At first they will have buyers for their homes to fund the migration...until they don't. Before that there will be a frenzy of wasteful government spending to try to preserve the regions and way of life. It will not work.

Climate change is the reason we keep digging up civilizations that "just disappeared without a trace."

The actual west coast is pretty low impact other than sea rise. If you are close enough for the breeze but far enough to avoid the storm surge with a 1.5m rise, that is your ideal. The reason is the cool/cold pacific current. There is no prediction that it will shift, and if it did, I'm not sure any models would tell us what came next. The rainfall issue is certainly a thing, but it impacts agriculture and housing further inland. The east isn't safe either, the extremes will get more extreme. Places that don't usually have A/C will need it. Altitude is your best bet. Buy on the west side of the rockies for the rain at ~7-9kft. Not that your deed will matter when everyone comes knocking.

Southern California's watershed (Los Angeles, San Diego, Inland Empire), extends 1,500 miles eastward, to the eastern front of the Rocky Mountains, watershed for the Colorado River.

One reason urban developments are so sparse and widely-separated west of the Rockies is that there's simply insufficient water, rights are held under Spanish-derived law (seniority), and those senior rights are held, many by large West Coast cities, most especially Los Angeles.

Climate changes across that entire region will have a profound impact on coastal dwellers, even if sea-level rise is modest and coastal climates are largely unchanged. Simply because that's not where the water is coming from.

(Northern California's water largely comes from Sierra Nevada snowpack (San Francisco, Central Valley), or local reservoirs (mostly coastal counties). Santa Barbara similarly relies on local sources, largely Lake Cachuma, a reservoir in the Santa Ynez Valley. Supplies can dip parlously low in drought years. <https://www.countyofsb.org/2795/Reservoirs>).