People are complaining about this, but it makes a lot of sense. Normally, you can't use reverse osmosis to turn salt water in to fresh water without some form of power to overcome osmotic pressure, and that's what makes reverse osmosis desalination a challenge to implement at scale.

Darth Vader is demonstrating that with the addition of The Force, it becomes possible to create the pressure required for reverse osmosis to occur without needing a battery or power supply, and thus, fresh water from the ocean!