From Cape Town to Tehran to Lima to Phoenix, dozens of cities across the globe have experienced water shortages recently. And in the next five years the world’s demand for fresh water could significantly outpace supply, according to a United Nations forecast. Now several companies are turning to an unexpected source for a solution: the bottom of the ocean.Called subsea desalination, the idea is to remove the salt from water in the deep sea. If it worked at scale, the technology could greatly all...