That won't work for Mexico anymore than it does for Southern California. The plants require massive amounts of energy and the waste water is super concentrated salt water which will kill marine life and throw the entire ecological balance out of whack. No one wants that. The inlet pipes would kill massive amounts of marine life as well so desalinization is a non starter except for on a small scale like Navy ships out to sea for extended duration's.
You are completely wrong on all counts. And trust me, it’s a good thing! Read up on modern desalination plants, such as the ones in Carlsbad, California and Tampa Bay. They are very successful, economical and quite good for the environment.