Send them desalinization equipment .
They have a big enough coastline.
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.