Energy from oceans is used as tidal energy, wave energy, and thermal energy. 

OTEC (Ocean Thermal Energy Conversion) power plants are devices used to harness ocean thermal energy. 

In OTEC system, the warm surface water is used to boil a liquid like ammonia or CFC. The vapour of the liquid is then used to drive turbine of generator. The cold water from the ocean depths is pumped up to convert the vapour again into liquid.