Whether it's a leaking hot water heater or just a leak under the sink, you want to catch water leaks just as soon as they start, before damage occurs.
The idea is simple enough: use extra long (30mm) male pin headers to elevate the detector above a surface where water might encroach. Water which shorts against any two adjacent pins should immediately wake up the detector so that it can report the problem. I chose LoRa because for any detected water leaks you definitely want highly reliable RF communication.
This water leak detector is powered by two AA batteries. As you can see from the BOM, not many parts are required to put this together. Also, the parts are inexpensive.