There seems to be an acceleration in plans to tap into the enormous energy naturally supplied by oceans, both tidal power and power from the deep oceanic currents. Check out a Scientific American article that begins to outline the plans to fast-track R&D in the American Northwest. The concept is completely analogous to wind farms, where 'windmills' are submerged into ocean current and 'blown' into spinning turbines just as terrestrial windmills and generators work. A challenge that becomes more of an issue in water is the effect on sea creatures of all types that benefit from ocean currents for food sources.
A second challenge that is relevant to both land-based and ocean-based mills and generators is the complete lack of infrastructure to transmit energy from the source to cities. This is precisely why the Obama administration dedicated down payments in the stimulus package for the initial development of some of this infrastructure, as well as money for modifications to the power grid in order to handle new energy plants and sources. As I tell my students who are now graduating high school and are interested in engineering and the physical sciences, it is a good time to be entering college and, in a few years, the job market, as we necessarily need to begin transforming the country's infrastructure and the way we produce energy. It is an unbelievably large task from both the research and applications sides of the business, but one in which we must accept and succeed as quickly and as efficiently as possible.