Water leak sensor works with Sonoff rf bridge @ AliExpress – $2.81
I ordered one of these to try out with the Sonoff rf bridge and it works fine. You will need an MN12 battery to power it (plenty on ebay and amazon). I’m going to put one in the bathroom and one in the kitchen. You could even make it switch a solenoid valve to turn off the mains water to stop a flood in theory. Also if you can solder you can change the sensor for a push button ( e.g. doorbell) or floor pad for an intruder alert, endless possibilities.