The cheapest method to produce drinking water from the sea is through a process called "reverse osmosis." Reverse osmosis is a water purification technology that uses a semipermeable membrane to remove ions, molecules, and larger particles from seawater. This process requires a minimal amount of energy, making it an affordable option for producing drinking water. Reverse osmosis systems are widely used in desalination plants, where they have proven to be an effective and cost-efficient solution for producing clean drinking water from seawater.