My old boss damned the creek running along one side of his property to create a fishing pond. I built him a little waterwheel that drove a small generator that lights his fishing dock. Nothing fancy, simple pvc paddles on a wood frame, chain drive to a little DC motor I had in my junk box. Lights up a little LED light at night. No battery, no electronics beyond a zener/current regulating resistor... Still running 17 years later! Way it is situated bearings stay mostly dry so lifespan has been great, replaced them once.