We can desalinate all the water we want in California
we already have a bunch of desalination facilities, and can build those out as far as need be
but desalinated water isn't as cheap as already-fresh water. That's mostly an annoyance for, say, residential use. But it's harder to competitively do agriculture on desalinated water. Israel has done some limited agriculture on desalinated water, but farmers are not generally going to be okay with that as an option. Most agriculture uses a lot of water.
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Someone ran the numbers to see roughly what it would cost if we did only desalinated water for everyone in the US.
https://hannahritchie.substack.com/p/how-much-energy-does-desalinisation
Producing enough drinking water for someone — assuming 3 litres per day — costs just $2.30 for the entire year. That’s less than the cost of a single bottle of water in many countries.
We actually use way more water than we do for drinking
we're pretty prolific with water to do things like lawns.
If the average person in the United States uses 310 litres of water per day for domestic uses, it would cost them $0.42 per day. Or $154 per year.
We'd probably want to conserve more nationally in such a scenario (Californians use less than the national average). But even with no conservation, it's very much doable.
It's a cost that we'd rather not pay if we don't have to, as a society, but we certainly are able to do so for residential needs.
But if there isn't enough freshwater to do agriculture, then who gets the freshwater determines which farmers go out of business (or at least have to shift to something else) and which get to keep operating.