Ocean Currents as Alternative Energy

What I learned is: ocean currents are gaining significance as a possible form of alternative energy. Water is dense, and carries an enormous amount of energy, that could be possibly captured and converted into usable energy. The way to capture this energy would be through the use of water turbines. Turbines convert water into energy.

My connections: the dams that I see from the highway, and they are really big, and when the water flows through them they make kind of man-made waterfalls. They are all used to create alternative energy.

I wonder: how they measure the energy in the water, and what makes them choose the rivers and waterfalls they dam, and also how much impact the dams and turbines have on the wildlife in the area.