You can use a humidity sensor to give an alert of boat leakage, or when your books start being flooded by heavy rain attack on your broken roof, or anything you can imagine. A simple humidity sensor (a humidity probe) can be constructed using two copper wires placed as close as possible to each other, but no touching. This simple circuit can be used to make your simple humidity detector. The circuit schematic diagram is shown below.
If the humidity increase, the probe resistance will drop and the Schmidt trigger formed by Q1 and Q2 will be triggered, and the Q2 collector become low, this condition will trigger the S-R (set-reset) flip-flop formed by U1A, U1B. After triggered, the relay will remain active until you reset the circuit by pressing the SW1 switch.