Use ocean waves to desalinate water
Ocean wave energy has been in development since the 1970s and is a broad set of technologies that have proven to work. Wave power is an interesting renewable resource in that it is truly high-power and extremely predictable over time. It can be though of as condensed wind power because the energy of rolling sea waves is the result of wind over vast distances of the ocean surface.
Much of the research into wave power has focused on generating electricity, but there has been an important conceptual shift taken by a number of smaller startups: Why not use that mechanical energy to directly desalinate water? This is a great approach because it has all the technical factors pointed in the same direction. For instance, most wave power systems already rely on a system that drives hydraulic fluid to move the energy - the main requirement for pushing water through a reverse-osmosis membrane is just that: hydraulic pressure. Secondly, the source of salt water is abundant to a system floating IN it! And third, by producing freshwater directly, there is no need to ponder the 'energy storage' question faced by most utility scale renewables. Fresh water can simply be stored in a tank.
(By the way, there are some other great examples of renewable energy use that have their factors similarly 'pointed' in the same direction: Using solar concentrators for thermal applications like recycling or processing, using hydropower to pump water into fields, and using wind power on top of tall buildings to circulate the air within are all great examples.)
The designs of wave power devices can be broadly classified along the lines of how they 'reference' their energy. A tethered buoy, for instance, references the sea floor. There are large systems that rock back and forth on the surface, and can be said to reference gravity via an internal weight or pendulum. There are also systems that are essentially long jointed floats that are bent by waves moving under them. These reference different parts of the wave.
Wave energy reference types:
There is a great entry in wikipedia that presents a breakdown of various systems. It's a cool read if you've never really thought about ocean waves as a resource before!
With fresh water emerging as the most desired of all the natural resources, we can expect to see desalination playing a larger role in agriculture and urbanization. I read a great article in Scientific American this summer that talked presented how Israel is perhaps leading the way, with many other desert nations following suit. Ocean wave power provides a clear path forward in terms of having a reliable deployable scalable system. In my opinion, the best system for large scale applications are the Wave-Wave 'surface attenuator' systems, which are essentially long floating hinged ship hulls. These can be towed into place and simply anchored, without the need for much in the way of deep-sea installation or underwater maintenance. Though most of the current projects focus on smaller scale wave-earth point-absorber buoy types, the wave-wave referencing type holds the most promise for economic large scale deployments in the future.
Links to Wave Power Desalination Projects:
Inhabitat article from October 2016 about startup company SAROS
Waste & Wastewater International from October 2013 about Australian wave desalination
Carnegie Clean Energy website for their CETO technology
Page of the now-defunct Pelamis project, which tested some wonderful surface attenuators http://www.emec.org.uk/about-us/wave-clients/pelamis-wave-power/