Ocean-Driven Hydropower

Take this with a grain of salt, as I really have no expertise relating to this. I'm merely voicing a simple concern that could be elaborated upon. From http://images.businessweek.com/ss/09/02/0225_inventions/2.htm:
"Ocean-Driven Hydropower
Till now, hydropower has mostly been generated at dams. Now, turbines around the world are being designed to harness the power of the ocean. Blue Energy Canada is close to commercializing a turbine that captures energy from ocean currents and already has purchase power agreements in India, Indonesia, and New Zealand. With a set of subway-size floating turbines, Pelamis Wave Power is converting wave power into electricity off the coast of Scotland."

Alternative energy sources are a wonderful idea, but with the growing concern about our effect on the climate, shouldn't we scrutinize all of our energy plans? Unless I am somehow mistaken, getting energy from ocean currents means impeding the currents to some degree. Don't these currents we wish to tap into play a significant role in how the climate all over the earth works? Aren't we risking another type of drastic effect on the climate by impeding the natural movement of the ocean? I am just not aware of how great the effect of drawing energy from ocean currents would be on their natural flow.