Fresh water on ships plays an important role for crew, passengers and for cooling of various machines and devices. Therefore, the vessel must be supplied with sufficient quantities of water. Fresh water is obtained by means of special devices, and then purified and stored. Today there are a number of different devices for obtaining freshwater from the sea, with different working principles. Which device will be built into the ship depends on the type, size, purpose of the ship as well as the available spare energy for the desalination drive. There are many methods of seawater desalination, all ranging from: seawater molecules away from seawater or extracting salts from sea water. Fresh Water Generator - A vaporizer is a device in which fresh water gets from the sea and is one of the most important devices on board. It uses the evaporation method, so seawater and heat are required for this process. Fresh water produced by a fresh water generator is used for drinking, cooking, hygienic, or even for other important devices that cool water is a coolant. Reverse Osmosis is a method used to obtain drinking water from salt water. The process uses a semi-permeable membrane through which pure water passes, and the salts are lagging behind. This process allows the removal of the smallest particles from the water. This provides a quality drinking water that meets all drinking water standards. Water retention or desalination is a process of reducing the mineral from water, ie reducing the levels of certain constituents (chlorides, sulphates, magnesium, calcium and carbonates) to the limits determined for the quality of potable and technological water. By reverse osmosis, the effect of desalinization is approximately 98-99%. A very important stage in water production is chlorination and correction of the pH of water, and its disinfection.