The ocean can produce thermal energy from the sun's heat and mechanical energy from tides and waves. Tidal energy can be derived from barrages, similar to hydroelectric dams, or from tidal flows, using devices similar to underwater wind turbines. Conditions are good for tidal power generation in both the Pacific Northwest and the Atlantic Northeast regions of the United States. Prototype tidal-power devices are under testing in the United States and Europe.

Wave-power devices extract energy directly from surface waves. Wave-power rich areas of the world include the western coasts of Europe and Canada, southern Africa, Australia and the northeastern and northwestern coasts of the United States. A wide range of device concepts are under small- and large-scale testing in the United States, Europe and Australia.

Ocean thermal energy uses the temperature difference between surface and deep waters to generate electricity. Good areas for application are tropical coastal areas, roughly between the Tropic of Capricorn and the Tropic of Cancer. There are numerous industrial sources of waste heat with comparable temperature differences, which would be less costly to exploit and nearer to the consumer.

Advantages
Ocean energy is renewable and generally unobtrusive to the public.

Issues
Tidal power plants that dam estuaries can impede navigation and sea life migration. Silt build-ups behind such facilities can impact local ecosystems. Tidal fences also may disturb sea life migration, and careful site selection is key to keeping the environmental impacts to a minimum. It also is key to keeping the environmental impacts of wave-power systems to a minimum if they are shore mounted. All these types of ocean energy require large investments. Device concepts are relatively immature compared to wind, geothermal and solar, and hence have a hard time competing economically against traditional sources of energy.