As might be expected, some of the energy companies using fossil fuels are investing in renewable energy sources. Statoil is one of them, with work on carbon capture and storage, wind power, and geothermal possibilities, as well as improved technology for current sources.
They have three research centers in Norway as well as a heavy oil research group in Canada. www.statoil.com

Canada is conducting research into the large tides effected by the gravitational pull of the moon and sun in the bay of Fundy. Large turbines (Atlantis/Lockheed Martin/Irving Shipbuilding) are planned.
http://www.nrcan.gc.ca/science/story/3616
Also see Blue Energy- http://www.bluenergy.com; tidal bridge technology.
Tha Annapolis Tidal Power Plant (Nova Scotia) was the first to transform tidal energy to electricity (Tidal Power Corp.). There are few places where the tidal range is sufficient, say Canadian scientists. One of the best in North America seems to be coastal British Columbia.
The Orkney plant (mentioned above in a post) is supplying power to the grid in Scotland.

Note: Currents like the Gulf Stream are slow and surficial; there is much deep water beneath such currents.