Apparatus and a method for the desalination of a liquid are described. In particular the apparatus and method are suitable for the desalination of water so as to provide a means for supplying potable water from an impure source. The described desalination device (1) comprises a water engine (3) and a reverse osmosis unit (4). The liquid to be desalinated is directed to the hydraulic rams (10, 11) of a water engine (3). Thus when located within a body of water, the operation of the water engine (3) increases the pressure of the liquid to a sufficient level that desalination can take place within the reverse osmosis unit (4). The desalination device (1) is particularly suited for incorporation with a body of water that exhibits tides since this provides a renewable source of energy for the operation of the water engine (3).
