This drawing illustrate a water detector. A water detector is basically a standard ADC reading between two resistors where the 2nd resistor is a PCB as illustrated above. As the water raises the PCB will start leading current and the ADC reading will move from 3.3V to GND. This is the same principle as used for temperature- and light detection resistors- except that the resister is a PCB in this case. We we can apply the sensor with a cable some meters from the actual sensor Logic as we don’t want electronics flooded.
A water detector is something you don’t believe you will need, but if you ever have a water leak you would like to know about it fast since water usually cause less damage if it removed instantly.