“Oneka’s floating desalination machines – buoys anchored to the seabed – use a membrane system that is solely powered by the movement of the waves.  The buoys absorb energy from passing waves, and covert it into mechanical pumping forces that draw in seawater and push around a quarter of it through the desalination system.

The fresh, drinking water is then pumped to land through pipelines, again only using the power provided by the waves.”

From BBC.