Water-driven electrostatic generators are really easy to build. The key is to have droplets separate at a point where there's an electric field, which causes the detaching droplet to carry a charge. Properly arranged this can cause an exponential increase in charge with time.
It might be a useful effect for small electronics in damp environments where solar isn't a great option. Caves, rainforests, showers.
https://en.wikipedia.org/wiki/Kelvin_water_dropper