The short answer is all those problems have already been solved.

Israel desalinates 75-85% of its drinking water. The problem is political and economic dysfunction.

California for example could be doing widespread desalination with nuclear power and technology from the 1970s. They could also greatly expand reservoirs and waterways, but don’t do it. Very similar to Rome in the 400s, when people were using aqueducts built by a past civilization but lost the ability to construct them.