It uses power from the engine to drive a pump which forces seawater through a very fine membrane at high pressure. The membrane only lets water molecules through. The resulting output of fresh water goes directly into our water tank, and the super salty water by-product goes back into the sea. It produces 120 litres per hour.
Without this machine we would have to limit ourselves to using 4 litres per day per person on the long crossing ahead. With it, we can even have a shower every day. Fantastic.