Tides never stop, with water moving first one way, then the other the world over.

http://www.openhydro.com/envEnergy.html

environment: tidal energy

Tidal streams are created by the constantly changing gravitational pull of the moon and sun on the world's oceans.

Tides never stop, with water moving first one way, then the other the world over. In certain areas geographic features such as headlands or channels serve to accelerate this movement of water.

Since the relative positions of the sun and moon can be predicted with complete accuracy, so can the resultant tide. It is this predictability that makes tidal energy such a valuable resource.