Energy in water, either in the form of wave motion or temperature differences, can be harnessed and used. Since water is about 800 times denser than air, even a slow flowing stream of water, or moderate sea swell, can yield considerable amounts of energy. 

There are many forms of water energy: 

• Hydroelectric energy is a term usually reserved for large-scale hydroelectric dams such as the Grand Coulee Dam in Washington State and the Akosombo Dam in Ghana.
• Damless hydro systems derive kinetic energy from rivers and oceans without using a dam.
• Wave power uses the energy in waves. The waves will usually make large pontoons go up and down in the water, leaving an area with reduced wave height in the "shadow". Wave power has now reached commercialization.
• Tidal power captures energy from the tides in a vertical direction. Tides come in, raise water levels in a basin, and tides roll out. Around low tide, the water in the basin is discharged through a turbine.  British company Lunar Energy announced that, in conjunction with E.On, they would be building the world's first tidal energy farm off the coast of Pembrokshire in Wales.
• Ocean thermal energy conversion (OTEC) uses the temperature difference between the warmer surface of the ocean and the colder lower recesses. To this end, it employs a cyclic heat engine. OTEC has not been field-tested on a large scale.