It finally happened to me - my hot water tank blew. As luck would have it, I actually had one of our water leak detection products installed and I was able to get the water shut off within 10 minutes, still had 1/2 inch of water in the furnace room to deal with.
Here is a pic of our wired water leak sensor pcb:
The processor is a PIC18LF14K22, and the circuitry also supports (you guessed it) a temperature sensor, an optional humidity sensor, an audible, and a dipswitch which is used to select a sensitivity.
Here is the actual water leak detection circuit:
The EN_H2O line is driven by the PIC IO line which turns on the 3.3 VDC to the J5 pins for about 1 second. The J5 and J6 pins are stainless steel screws about 1" apart. The V_H2O line goes to an A/D pin on the and the voltage detected easily translates to a reading of water conductivity.
This install saved us thousands of dollars worth of damage if we had not bee notified in time. The rest of the system consists of a wireless gateway and "Enviro Cloud" web based software (see www.vpprocess.com).
Here is a pic of the detector with it's case on:
And here is our wireless version of the water leak detector:
Water leak damage is one of the leading insurance claims and causes billions of damage every year. I was glad I dodged that one :)