Also called hydrokinetic energy, wave power is another answer. According to the Union of Concerned Scientists, the kinetic energy in waves, river currents and tides in the United States could provide energy to more than 67 million houses.
There currently exist two different types of machines that can extract kinetic energy from water: rotating devices and wave energy converters. Rotating devices operate fairly similarly to hydroelectric devices, in that the power of the water's motion turns a turbine and rotates a shaft inside a generator, and electricity is created. This technology is also similar to what is found in wind turbines. Some rotating devices look like underwater wind turbines, and others look like eggbeaters.
There are several different types of wave energy conversion devices. What all of these WEC technologies have in common is that two or more objects move in relation to one another. The waves move the displacer, while the reactor responds to the displacer's movement. There are four common types; the first is the oscillating water column. Waves move in and out of a collection tank from underneath, and the movement in the water level pushes air against a turbine, and the turbine turns to produce electricity.
The second type is called a point absorber. This takes the up-and-down movement of the waves to add pressure to seawater or another fluid, and this pressure turns a turbine.
The third type is called a heave-surge device, or an attenuator. It floats parallel to the direction of the waves and literally builds electricity while riding them. The movement of the structure actually adds pressure to the hydraulic piston that pushes fluids into the motor, powering a generator.
The last type is called the overtopping device. This creates a floating reservoir with a higher head of water than the surrounding waves, and this difference in height makes the pressure needed to rotate the turbine.