In Roman Britain (and at least one place in medieval Ireland), they used a simple water clock consisting of a thin metal bowl and a bucket of water.
You filled the bucket to the same place every time. Then you put the bowl on top of the water. Drilled into the middle of the bowl’s bottom was a tiny, tiny hole. As time passed, the bowl slowly sank into the water. Once the bowl sank completely, you knew that the given time had passed. This kind of clock was used in Persia by villagers to time the use of irrigation canal water to particular plots of land.
This is called an “inflow” type of water clock (or “clepsydra”).
The “outflow type” is when you have a bigger vessel filled with water, and you have holes to let the water out.
With both types of clocks, you can have lines to show how much time has passed when the water has gotten to a particular line.