Whenever there’s a drought in California, a seemingly obvious source of new water supply beckons. The state abuts a giant ocean. Why not just take the salt out of some of that seawater? It’s the high-tech, forward-looking thing to do, right? It’s also the really expensive thing to do. Of all the options for increasing the state’s water supply considered in a report out Thursday from the Pacific Institute, an environmental think tank based in Oakland, California, desalination costs the most per acre-foot (325,851 gallons, 1.2 million liters) of water produced.