A low boiling, water immiscible medium is directly contacted in liquid state with relatively hot or warm water (e.g. with hot recycled fresh water in a desalination system or with warm surface sea water in an energy production system) and in vapor state with relatively cold or cool water (e.g. surface sea water in a desalination system or deep sea water in an energy production system) whereby thermal energy is efficiently transferred between the phases that are so contacted; the energy transfer is for the purpose of evaporating the immiscible liquid in an energy production system and is for the purpose of evaporating saline water in a desalination system; and the effluent from the process which is returned to its natural environment (e.g. to the sea) is treated to remove all significant amounts of entrained and dissolved immiscible medium.