A simple but effective Grove water sensor. Expose the electrodes to a drop of water, and the signal drops very low. Can be used with both analog and digital ports.
It's a simple and effective concept for water detection. Use a grid of electrodes, with one half attached to a very weak 1M pull-up resistor. When a water drop bridges the electrodes, the resistance falls dramatically - so much so that this Grove module can be plugged into either an analog or digital port.