Desalinating ocean water for their country's needs. Florida has two ocean desalination plants, one of which has been in use since the late 70's.
Cali has or has had desalination plants in the planning stages, but I don't know of any that are being built off the top.
The larger cities in Australia rely on desalination plants for much of their water. Some Middle Eastern countries rely on it, with the largest desalination plant being in Saudi Arabia.
I did a google search on this some years ago on the IC and posted it up, complaining about the same thing: The technology exists, and has been successfully been used for decades. California is just on some fuck shit and would prefer to put a tremendous strain on the Colorado rather than use the ample water resource they already have; the Pacific.