The wars of the future will be fought over it! Clashes happen even today here and there by small factions for control of areas that have a source of freshwater. That’ll be a sad time to be alive. If an cost effective source of energy could be used such as tidal turbines or something, I think desalination of ocean water via a process called reverse osmosis would work. But like you state above, the problem would be providing for the overwhelming number of people