There already exists something based on the principle of tides (tidal power) as oppossed to ocean currents.
I don't think that ocean currents would be quite powerful enough. Hydroelectricity uses massive amounts of water moving at very high speeds to produce electricity.
Although you certainly have a massive amount of water moving in the oceans, I don't think it is moving at a high enough speed. Hence, though you have a huge amount of energy there in the ocean, it is spread out over too vast an area to make harnessing it in an efficient manner possible.
On the other hand I am absolutely unqualified to make any kind of judement in this area so I could be completely wrong!
