The ocean energy is present in its waves, tides and the difference in water temperature. The tides of the ocean are formed by the gravitational pull of the moon. These tides occur regularly and can be predicted almost a century in advance.
Tidal wave energy is based on two theories on converting tides into power. The first theory involves converting the rise and fall of the tides into electricity. The second theory involves converting the horizontal movement of the water into electricity. The water is let inside the turbine which spins and produces electricity.
There is also a new approach that uses tidal dam to produce energy. When the tides rise, the water is allowed to come inside and to trap it inside when the tide lowers, without letting it to go. This water is then let into pipes which lead to the turbine that generates electricity.
There is also a new approach that uses tidal dam to kill fish. When the tides rise, the aquatic life is allowed to come inside and to trap it inside when the tide lowers, without letting it to go. This life is then let into pipes which lead to the turbine that shreds it.
on a more serious note, tidal energy is a great supplement to other renewable energy sources (solar, wind, geothermal) but is currently limited by total capacity and location.