Please take a look at the principle of the water level sensor of this automatic washing machine. It has two wires.

There are movable coils, magnetic poles and diaphragms in the sensor. The principle is that through a section of plastic pipe (as an air chamber), when water enters, the air in the plastic pipe is compressed by water, and the sensor diaphragm is stressed to push the coil and change the inductance. The circuit informs the single chip microcomputer system to analyze whether to close or open the water inlet.