Hitting desalination plant will mostly (and almost only) hurt the population outside of IRGC power circle, exactly the population the USA incited to rebel for a few years, with pakistani help. Basically South Alborz. Iran did have a water crisis, but it was agricultural water that made them have to import more food, not an issue of drinking water. The only way to truly start a global Iranian famine (what a terrible weapon tbh, i can't help but to think less of people who think about using it, it's impossible for me to stay neutral,it is an inhumane stance, and i would be ashamed of having it) is to block Iran northern trading routes to Russia/Kazakhstan/Azerbaijan, which, to be clear, is impossible.