ocean energy or ocean power is the energy carried by ocean waves, tides, and ocean temperature differences. The movement of water in the world’s oceans creates a large amount of kinetic energy, or energy in motion. This energy can be harnessed to generate electricity to power homes, and whole industries.

Well in short "ocean energy can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves. Oceans cover more than 70% of Earth's surface, making them the world's largest collector of solar energy . The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a little amount of the heat trapped in the ocean could power the entire world."

This is how. "There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems."

"Example of the way we would generate the energy Kinetic energy (movement) exists in the moving waves of the ocean. That energy can be used to power a turbine. In this simple example, the wave rises into a chamber. The rising water forces the air out of the chamber. The moving air spins a turbine which then turns a generator."

Pros

The power density of ocean energy sources are 800 times than of wind power.

Wave power, and especially tidal resources, are a more predictable source of electricity than solar or wind.

Less capital cost per unit of electricity generated if compared to other renewables.

Cons

Like wind power, waves and tides, the most popular ocean energy sources are still variable.

Primary downside for marine renewables is the unknown operations and maintenance costs, possibly two to four times higher than solar or onshore wind.

A lack of commercial projects on-line today.
Too many designs currently competing for funding due to a lack of consensus on best engineering practices.

"The environmental impact of wave and tidal energy is still pretty much in unknown territory, simply because these two energy sources are yet to move beyond the stage of prototype projects used for demonstration purposes and become widely deployed technologies."

The possible negative environmental effects of tidal and wave power include:
- the frames of the turbines could lead to disruption in movement of large marine animals and ships through the channels on which the barrage is built
- construction of tidal power plant can also disrupt fish migration in the oceans, and even kill fish population when passing through the turbines.
- the possibility of noise pollution

(http://hydroearth.blogspot.com/2012/11/wave-and-tidal-energy-environmental....)

This is how we use thermal energy from the sun and ocean in order to produce energy