Sensor, Water In Fuel: Operation
WARNING: This page is about a different car, the 2017 Jeep Cherokee. However, it is still accessible from the selected car via links, so may be relevant.
The non-dielectric properties of the water allow the sensors probes to close the electronic circuit when water is present in the system. When diesel fuel is present in the system, its dielectric (does not conduct electricity well) properties do not allow any electrical contact between the probes. A 12 volt power signal is always present in order to illuminate the indicator on the instrument cluster.