Producing fresh drinking water from the sea - desalination - has always seemed to be the most obvious answer to water shortages.Our oceans cover more than 70% of the earth's surface and contain 97% of its water. But the energy needed to achieve this seemingly simple process has been costly. Now, thanks to new technologies, costs have been halved and huge desalination plants are opening around the world.The largest seawater desalination plant ever, Israel's Sorek plant near Tel Aviv, just ramped up to full production. It will make 624 million litres of drinkable water daily, and sell 1,000 litres -...