There are several methods to generate electricity from the ebb and flow of tides. Tidal barrages are the most common; they are basically dams, usually built at the entrance to an inlet. Water flows in and out through sluice gates, moving turbines.
The Rance Tidal Power Station in Brittany, France, has been in operation since 1966, and was surpassed as the most productive of its kind by South Korea's Sihwa Lake Tidal Power Station in the mid-1990s. Although the sluice gates are designed to be a porous barrier, they still change water levels in estuaries and pose environmental problems similar to those caused by large dams.
Tidal fences, with vertical axis turbines, are cheaper and less damaging than barrages, but can be disrupting to large marine animals. Tidal turbines work just like wind turbines; individual poles are embedded in the seabed.