I’ve been pondering the same thing since day one...seawater!
As you stated, introducing seawater into a municipal potable water system would open a can of worms, but...way better than zero water at hand for fire fighting.
I thought about dedicated fire suppression reservoirs up in the hills solely supplied with seawater as backup. A seawater pumping station at the shore connected to the reservoirs with a 12” main ought to do the trick...over time of course. There would be a substantial pumping head to overcome maybe needing a booster station part way up.
But then there are the seawater lines from the reservoirs servicing the non-potable, “fire only” hydrants. Oy...lots of infrastructure there, and I haven’t figured out how to pull it all off...yet.
Pay me $750K, and I’ll work on it. 😀
LOL !!! I will bid $749.00.ðŸ¤
Considering all the money that California spends on liberalism, one would expect them to have whatever is needed to deal with this.
But nooo...