Update from Bracken this evening per my email request:
Basically, water desalinization = reverse osmosis = “RO” = pushing dirty/brackish/saline water through a microfilter. (Think about Gore-Tex, the miracle fiber of the 1980s, but using electricity.) The potable water is forced through onto the good side, where you can drink it. The bad side of the filter gets clogged up with varying degrees of crap, so over time the filters must be cleaned and/or replaced. These are factory-made, you cannot duplicate them years into SHTF. When they fail, they are done forever.
And it all takes electricity, except for the manual “life raft” models, where it takes about an hour of hand pumping to obtain one cup of potable water out of sea water. On mega yachts, scaled up, lots of diesel generator electricity is turned into enough fresh water for supermodels to shampoo their hair. In Saudi Arabia, scaled WAY WAY up, (sitting over petroleum lakes), enough fresh water is generated for millions of people. This could be done in California, but only with lots of new nuclear power plants, which they will never build.
This information off the top of my head is all just for a general ballpark understanding of “desal”, meaning desalinization. Dirty salt water takes the most power to convert to potable, with slightly brackish but “clean” river or lake water taking the least. Your RO filters will also last MUCH longer, but as in most things, YMMV. From my POV, IMHO, it’s much better to move to a zip code where you are living right over potable ground water with a simple hand pump well, or at least a water well that is run run off solar power with dedicated old-school batteries.
This is my backyard SHTF water solution, here in North Florida. Potable water is just 40 feet down. I can fill a five gallon bucket with potable water in about a minute, if I move that handle fast. The parts that wear out can be replaced or fabricated at home, unlike high-tech RO filters. This is just my experience, for your accumulation of hopefully helpful information.