An in situ desalination apparatus comprising a reverse osmosis unit (15) having a reverse osmosis medium, the reverse osmosis unit in use to be located within a body of water under ambient bio-physico-chemical conditions often different from those at the surface, the unit having an inlet (17) opening to one side of the reverse osmosis medium, in use the inlet to be located below the upper surface of a body of water, the unit having a concentrate outlet (25) opening to the one side of the reverse osmosis medium and opening into the exterior of the unit at a position spaced below the inlet to return concentrate into the water body, the unit having a permeate outlet (27) opening to the other side of the reverse osmosis medium, the permeate outlet communicating with a delivery line extending from the body of water, a pump (23) between body of water and the reverse osmosis unit and adapted to pressurise the water located at the one side of the reverse osmosis medium.