Tides, waves and currents can be used to produce electricity. Although still at the research and development stage and not yet commercially available, promising ocean technologies include:
Wave energy, whereby converters capture the energy contained in ocean waves and use it to generate electricity. Converters include oscillating water columns that trap air pockets to drive a turbine; oscillating body converters that use wave motion; and overtopping converters that make use of height differences.
Tidal energy, produced either by tidal-range technologies using a barrage (a dam or other barrier) to harvest power between high and low tide; tidal-current or tidal-stream technologies; or hybrid applications.
Salinity gradient energy, arising from differing salt concentrations, as occurs where a river empties into an ocean. Demonstration projects use "pressure retarded osmosis", with freshwater flowing through a membrane to increase the pressure in a tank of saltwater; and "reverse electro dialysis" with ions of salt passing through alternating tanks of salt- and freshwater.
Ocean thermal energy conversion, which generates power from the temperature difference between warm surface seawater and cold seawater at 800–1,000 metres depth.