Sea and ocean currents are a stable renewable resource that transfers a relatively large quantity of energy. Various types of turbines quite reminiscent of wind turbines are used to utilize this energy. Due to the greater density of water, however, they turn much more slowly, and hence pose no danger to aquatic life.

Tidal power plants are based on the simple principle of collecting water in a reservoir during when the tide flows in and releasing it via a barrier equipped with turbines as the tide ebbs. These power plants have a relatively large effect on their surroundings, because by damming the estuary they restrict fish migration and marine transport, and accelerate the accumulation of sediment in the estuary.

Ocean wave energy has huge worldwide potential. Systems for generating energy from waves convert their movement into useful work, which is then converted by various generators into electricity. The best-known water energy converters utilize various floats or an oscillating water column that compresses air, which then drives air turbines.

Tidal turbines are similar to wind turbines - flowing water turns their blades that turn their rotor that turns the generator. Because water is about 800 times denser than air, tidal turbines have to be much sturdier and heavier than wind turbines. They usually work with variable low-head, so some sort of variable-blade Kaplan turbine is usually employed. Turbines for tidal power are relatively slow-turning, usually in the range 50 to 100 rpm.

Plants with two-way generation use a bi-directional turbine, while other types use a bulb turbine. In the bulb turbine, the generator attached to the turbine shaft is housed in a watertight pod, or bulb, directly behind the turbine runner. The whole turbine-generator assembly is then hung inside a chamber that channels the water flow around the bulb to the turbine blades to extract the maximum amount of energy as possible. Tidal plants often have a series of small turbines running along the barrage because these can exploit the available energy more effectively than a small number of large turbines.

A concrete dam enclosing an inlet of an ocean bay or lagoon that forms a tidal basin. The bottom of this barrage dam is located on the sea bed with the top being just above the level of the highest annual tide. The barrage has a number of underwater tunnels cut into its width allowing the sea water to flow through them in a controlled way by using sluice gates on their entrance and exit points. Fixed within these tunnels are huge tidal turbine generators that spin as the sea water rushes past them either to fill or empty the tidal reservoir thereby generating electricity.

Watertight gates closing tunnels in a barrage so as to enable the tidal basin to fill and empty through ongoing tides. Sluice gates are closed at both ends of tunnels with turbogenerators. Several types of gates are available, e.g. radial gates or vertical-lift wheeled gates.

Suitable bays can be closed by a barrage and become a tidal basin. Places for tidal power plant construction need to have such geometry that ocean levels rise is high during flood tides. Closing a bay can have environmental consequences - currents and sedimentation in the location will inevitably change, which could lead to changes in marine ecosystems.

The gravitational pull of the Moon and Sun along with the rotation of the Earth create tides in the oceans. When a tidal wave reaches the coast, in conveniently shaped bays the sea level rise could reach several meters. The highest global tidal reached, in the Bay of Fundy in Canada, is 15.8 m. High tides occur 12 hours and 25 minutes apart, so the production of a tidal power plant is perfectly predictable but not continuous.

There are three ways to operate a tidal power plant:
- Ebb generation, the most usual and effective way. During flood tide, sluice gates are opened and water fills the tidal basin. Then the sluice gates are closed. As the ebb tide slowly begins, the tidal basin can be more filled by the river flowing into the basin. When the head between the basin side’s and ocean side’s is high enough, sluice gates are opened and water flows back to the sea under the enormous force of both gravity and the weight of the water in the reservoir basin behind it, generating electricity. - Flood generation. During the ebb tide the sluice gates are opened and the basin is emptied. Then the sluice gates are closed while the power plant waits for the flood tide. Then it opens the sluice gates and water flows in the basin, generating electricity. - Two-way generation. Electricity is produced in the middle part of flood tide and also in the middle part of ebb tide. Between this the sluice gates need to be closed in order to create sufficient difference between the ocean side’s and basin side’s sea levels.

A marine turbine is similar to a wind turbine – the force of flowing water turns the composite blades, which turns a rotor connected via a shaft and gears with an electric generator which produces electricity. These turbines have a patented feature by which the rotor blades can be pitched through 180 degrees allowing them to operate in both flow directions – on ebb and flood tides. They start to rotate when the current is faster than 1 m/s and at maximum speed, the tips move at approximately 12 m/s, which is approximately 1/3 of the wind turbine’s average speed.

A crane for mounting the cross beam with turbines.

A horizontal arm mounted on a supporting pile with turbine and generator on each side. The cross beam can be lifted above sea level for easy access and maintenance.

Lift legs run either side of the pile providing the capability to lift the cross beam and rotors above the surface for maintenance.

A tubular steel monopile anchored to the seabed, with the upper part around 9 meters above average sea level. A cross beam with two turbines is mounted on the pile and can be moved vertically.

Marine current turbines exploit strong currents usually occurring in the straits where water flows very quickly. They can use ocean or tidal currents.

Air in the pneumatic chamber flows back and forth because of an oscillating water column, but the electric generator has to rotate only in one direction. This is solved by using a Wells turbine, which, once in motion, maintains the same direction of rotation, no matter the direction from which the air is flowing. The turbine consists of a number of symmetrical airfoil blades located around a hub. Their chords lie in the plane of rotation (i.e. no pitch angle). This gives the turbine the property of being able to rotate in the same direction, regardless of the direction of air flow.

The electric generator is directly connected to the turbine by a shaft.

The air valve allows closure of access to the turbine if needed.

A power plant using the energy of waves has to withstand harsh weather conditions and resist strong surf during storms, therefore it has a massive reinforced-concrete building.

The front wall slopes beneath sea level, where the opening allows entrance to waves which flow into a large chamber, also called a pneumatic chamber. The repetitive motion of the waves causes the sea level inside the pneumatic chamber to sink and rise and thus suck and blow air through the turbine located behind the chamber.

As waves crash against the front of the structure, water flows into the chamber, causing the water level to rise and sink. The air above the water in the chamber is forced back and forth through the turbine causing the blades to spin. The turbine is connected to a generator that turns the rotational energy into electrical energy.

Wave power plants are located in places where there are almost always waves braking on the shore and so can produce electric energy almost continuously.