Jon, our prolific administrator posted "Yeah, I always stick a $10 battery-powered water alarm in under-sink cabinets. Learned that lesson the hard way, but at least I learned it".
That prompted
Quote Originally Posted by Frank S View Post
I have alarms for carbon-monoxide smoke detectors gas leak detectors electrical fault sensors, and now it seems that I might want to invest in water detectors. Thanks Jon I hadn't thought of them.
Yes, if placed in a pan under a drain.

I'll endorse them too. As $10 the going price, far better than same amount wasted on an insurance policy. Sure the detector is reactive, but what alarm isn't? Rather clean up a 50 gallon spill or 500? There should be a business opportunity coupled to a smart water control valve....
Most common in basements. I'll suggest them in drain pans surrounding inside air conditioner/ furnaces too, announcing sump pump failure.

With the components about, making them is simple. A small sealed 'pontoon', 9v battery and horn, two leads exiting the side. Water completes the circuit and she wails. Assembly needs to be light and stable enough to float, keeping the leads in contact with pooling water.