"Water, water everywhere, and nary a drop to drink." Old cargo ships from years back were required to carry large supplies of fresh water to sustain the crew on a long voyage. A ship that had run out of drinking water would be carrying a sad crew indeed. Today, most commercial ships are equipped with a freshwater generator, a type of evaporator that converts sea water into drinkable, fresh water.

The concept of a freshwater generator is simple; sea water is evaporated using a heat source, separating pure water from salt, sediment and other elements. Freshwater generators often use the diesel engine jacket as a heat source, although steam can also be used as a heat source. Because freshwater generators often use existing heat to run, the cost of operation is low.

There are two main elements in a freshwater generator, one heat exchanger evaporates the sea water, and another condenses the fresh water vapor into drinking water. In the condenser element, the vapor is condensed through cooling, often simply using cold seawater to cool the outside of the unit.

The freshwater generator should include a feature to monitor the salinity of the processed water. If the salinity exceeds a specified level, usually between one and ten parts per million (ppm), the freshwater generator will automatically return the water to the feed line and put it through the cycle again.

A freshwater generator is increasingly used in desalination plants, typically located in coastal areas, which provide fresh drinking water to the local community. These have become very common in the Middle East, and are also in use in some California communities. More desalination plants are likely to be built as a response to drought, and concern over availability of water in growing cities. However, the costs involved in desalination plants are high, and are considered by communities only when faced with water shortages from traditional sources.