But as an overall plan, it's not so nuts (except for the energy cost, and the hundreds of miles of aqueducts that need to be built). The point about the upper states getting water out of this is a valid one, and Lake Mead supplies Arizona as well as California.
One downside to this plan: You'd kill the Grand Canyon.
~61% Imperial Irrigation District;
~20% Metropolitan Water District of Southern California;
~8% Palo Verde Irrigation District;
~8% Coachella Valley Water District;
Out of these, only the Palo Verde Irrigation District is inside the Colorado River basin. The Metropolitan Water District of Southern California has access to the coastline, so it can mix in desalinated water directly. It currently mixes in local water sources and water delivered from Northern California [2].
The Imperial and Coachella valleys require an interbasin transfer in either case, but despite the presence of canals and pipelines from the lower Colorado River to these sites, supplying them with additional water from the coast would require significantly less infrastructure than pumping said coastal water all the way up to Lake Mead.
[1] https://www.usbr.gov/lc/region/g4000/4200Rpts/DecreeRpt/2018... [2] http://www.mwdh2o.com/AboutYourWater/Sources%20Of%20Supply/P...