Too many people using water unwisely.
Watering lawns to the point where they look like Kentucky
Failing to recognize the role of biomass as Loggy pointed out.
That's just three but they might be the top three.
Absolutely! The best solutions, IMHO, have to be on the front end. Shut down the swimming pools, the golf courses, the fountains. Stop growing any crop that can’t be grown with a measurable amount of water - and if that means we go back to eating produce when it’s in season rather than watering it year round, that’s fine.
Regarding desalinization - there was a lot of talk about building a desal plant on the Colorado where I lived 20 years ago. I moved so I’m not sure what happened, but it got a lot of pushback back then. I hope it’s financially more feasible now.
One other note: for a long time, we’ve been using so much of the Colorado River water that we’ve been unable to pay our water debt to Mexico. We also need to be aware of impact on the Grand Canyon - failing to do that is how we lost Glen Canyon.
Oldie but a goodie: Marc Reisner’s ”Cadillac Desert:The American west and its Disappearing Water.”