Whereas wave power generates electricity in small, continual amounts through the rising and falling of the waves pushing air in and out of a turbine, tidal power produces electricity in a larger amount but only as the tides ebb and flow. It does this in one of two ways:
Tidal Stream Turbines
These work on the same principle as wind turbines, but instead of airflow turning the propeller blades they are rotated by the flow of water as the tide comes in or recedes (whilst some may be fixed in place, some tidal turbines can rotate 180 degrees to take advantage of the tide coming in and going out).
Tidal turbines are one of the few examples of renewable sources of energy which are not visually intrusive as they will be hidden beneath the water, which unfortunately cannot be said of their wind power relatives on land which have to be tall to capture the wind and so can be seen for miles around.
Tidal barriers are built right across a river estuary. The flow of water through small gaps as the tide goes in or out drives a turbine which generates the electricity. Lock gates allow ships to pass so that the tidal barrier does not completely block up the waterway with it being built right across.
Unlike tidal turbines, tidal barriers have large sections built out of the water which means that they can spoil the look of the area.
For both types of tidal power, the initial cost can be high to construct, although in reality all forms of electricity generation are expensive undertakings which take large amounts of time and money to create. Once up and running though, both tidal turbines and barriers require no fuel to be bought as the source of energy is free and unlimited.
Another disadvantage that both have is the potential negative environmental impact upon fish and other aquatic creatures living in the water, particularly tidal barriers which can block the progress of migrating salmon or cause injury and death to any creatures unfortunate enough to become entangled in the underwater turbine.
The underwater location makes installation and maintenance of the equipment more difficult, time consuming and costly than it would do if it were out on dry land where it is easier to get to. Also related to location, electricity generation reliant upon tidal energy can only be cited in certain locations where it can take advantage of an incoming or outgoing tide, which means other forms of renewable energy must also be built if the total is ever to make a serious dent in the reliance upon high-polluting fossil fuel-burning power stations.