My father had a leaking pipe accident last year, and it happened while he was away from his place for a week. The water was flowing for few days before he returned and that caused damage for thousands of Euro, not to speak about the inconvenience. To minimize the damage of such possible future events, I have set up a flood sensor. It is a very basic setup – a veroboard of two pads soldered to the interrupt pin of a Funky v2 and GND respectively. The interrupt pin has the internal pull-up enabled [EDIT: using external pull-up resistor will provide better power savings, 300K-470K will work fine] , this when the veroboard pads are dry, it would read HIGH, and if they are wet, it would read LOW:
The leads of the sensor are left long, so that the sensor itself is on the floor at the lowest point in the house, while the Funky v2 stays higher (glued to a wall) than possible flood waters. The sketch is simple, sleeps most of the time and wakes up every 12 hours to report its battery level. If a flood occurs, it wakes up the node immediately and it reports a flood event. Emoncms will send me event notification and I can take measures for both flood event, low battery of the node and lost RF communication with the node.