In a damp environment any sensor you use must be hermetically sealed anyway, whatever type it is, unless
you can bring the electrical part of it out to a ventilated area.
I suspect its quite important the sensor you pick will fail, if it fails, by falsely indicating flood, so electrodes
in a protective open tube might be good - any kind of damp and you'll tend to get leakage current, but you'd never see a completely open circuit with two wet electrodes. No moving parts to sieze up, and using it is simple, just
apply 5V through a 100k series resistor and measure the voltage across the resistor (in effect
the current through the resistor by proxy), and threshold. Remove the voltage when not actively
measuring to prevent electrolytic action. A capacitor across the resistor can remove any ac or RF pickup
signal.
You can test the thing easily in situ (using a cup of water).