Ebbing tides, ocean currents and crashing waves are all easy enough to understand, but it's a bit trickier to see how osmosis fits into the future of hydropower. When seawater and river water are separated by a membrane that only the latter can cross, osmosis naturally pulls them together. The resulting brackish water flows with enough kinetic energy to generate electricity.
If that seems a bit complicated, imagine trying to make it actually happen. But the last few years have seen great progress in the technology, notably the development of a membrane cheap and dependable enough for commercial use. The upside is that time of day, weather and the season play no part in the process, meaning everything is easier to control. The downside is that salinity levels in bodies of water are inevitably changed, making it harder for fish and other marine species to survive.