Nonexistence of water is not a big issue for 99% of fire fighting. It's having sufficient water pressure in the exact location where the fire develops that's nearly impossible. Forests are big, you can't put hydrants in them. Other fire suppressants dropped from planes tend to work better than dropping water from a plane, though some of both is best.
Most of California's fires happen nowhere near the coast, and in areas with plenty of fresh water (because deserts can't grow fire fuel). You can see the fire scars going right up to the edges of the lakes. Sometimes a plane will take water from the lake to use to fight the fire, but that's just one tactic, and it's not essential: https://en.wikipedia.org/wiki/Aerial_firefighting
And it's not that desalinization is all that expensive for what it does -- it's that it's impossible to compete with the price of just using fresh water that's already sitting there waiting. And the unfathomable amounts of water we want to use for things.
"Many people need desperately to receive this message: 'I feel and think much as you do, care about many of the things you care about, although most people do not care about them. You are not alone.'" ― Kurt Vonnegut