While it’s impossible to know which of these methods will bear more fruit than others and what technologies will be invented to take advantage of new sources of hydropower (water displaced by whales? cannonballs into swimming pools?), it’s a safe bet to say that in the future, more of our energy will come from the 70 percent of the planet covered by water.
Environmental considerations have to be weighed carefully, especially given that a major motivation for developing new energy sources is to wean ourselves of fossil fuels. But my bet is that in five to ten years, tidal pools, currents like the Gulf Stream and breaking waves around the seven seas will be keeping the lights on.