The Tides
When the moon is on one side of the Earth, the gravitation attraction pulls the oceans towards it. The sun exerts its own force on the ocean, which has a different effect depending on its location.
The sun and moon combination cause the oceans to change to an oval shape around the Earth, rather than a sphere. As the moon moves around the Earth, this oval moves round, and we have high and low tides.
This animation should make this clearer:
So, this is what causes the tides, and why water flows both direction along tidal rivers. This is where the story gets interesting, and tidal turbines come into the story...
Tidal Turbines
Water moves back and forth along tidal rivers as the tides move in and out. This moving water can be used to spin turbines placed in the river. These turbines look very much like wind turbines, and can generate energy regardless of which direction the water is flowing in. I have another lovely animation to show you...
Animation from Marine Current Turbines
Cool, no? All we see above the surface are those small markers buoys, and the turbines just get on with it out sight. As long as the moon is orbiting the Earth and the tides flow, we'll have power.