Primarily for those in possession of water creaks, waves or streams.
If you can divert the water, you most likely will use a turbine. Small scale installations are using low-voltage generators (12V). This may then be used directly or stored using ordinary batteries.
Shown above is a turbine module generating a maximum of 500W at 12V.
N.B! Check with your authorities about regulations and permits!
Wave energy harvesting demands for more extensive installations. The generator used must be extremely low-frequent or be using a gearbox. Experiments are being carried out, most promising is using wave energy to mechanically pump water to a higher level, using a turbine to generate electricity when letting the water back down. Demands for hefty investments at startup.