There are large parts of Nevada that could be 'reclaimed' (in the Bureau of Reclamation sense) without too much effort -- just a couple of well-placed dams, berms, and canals. Large parts of the state are in closed basins, from which the water just evaporates anyway, so using it first within the basin would make sense. Extensive projects like this were once the norm, but fell out of favor as the 1970s transitioned into the 1980s, feeling the impact of the environmentalist movement, the opposition of Jimmy Carter, and the failure of the Teton Dam.

Some big urban areas already need to rely on interbasin transfers for their water supply: LA, NYC, San Diego, Mexico City, SLC, Denver. Tunnels for water would actually be a sensible and 'boring' pivot for the Boring Company.

Why not both?

Also much, much easier to solve, and more than possible with existing technology. May require more clean energy but it can be solved.