Here's a simple grist mill. The first STL is the building and pond. The second STL is the wheel and pins. Glue the pin into the hole on the side of the building. Slide on the wheel and then glue the end cap onto the pin. The wheel needs to spin freely.
You'll need a small recirculating pump that can fit in the trough behind the building. Run the outlet hose from the pump up to the connector at the back of the sluice above the wheel. Depending on what type of filament you use, you might want to paint the entire structure first to make sure it's water resistant. Then fill the pond with water.
The denser the infill, the more durable it will be.