I have looked at water clocks (clepsydra), made through the ages. The earliest were used in Persia and Egypt. The water was measured as it flowed from one vessel to another. They became more intricate over time, relying on water pressure and a constant flow from a source usually the river.
Water could continually flow through this mechanism and exit, back into a river.
I will not have a constant stream of water from a renewable source, such as a river. The water levels need to be carefully controlled to make the most of the water flowing through, yet have the energy to splash.
The tipping bucket rain gauge
The tipping bucket used by meteorologists, I think could be adapted to fit in the base of a vessel and tip a quantity of water with enough energy to cause a “splat” .
Small model 10 cm length, easy to attach to a base plate with a gap at the bottom to allow the bucket to tip. A weight on one end to make sure it raises when the water runs out and so is ready to refill again.