Here's what you do. Skip the detector, Just put the whole laptop in there. when it goes offline, You have water in the sump.
Anyone can build a working sump flood detector. But it takes a real engineer to build one that can detect two floods.
"...And so that's how I ended up with a $14,000 basement repair. Yup, those damn chromebooks are more waterproof than you'd think! I mean it makes sense, they're designed to be vomited on by children all day at school."
Very much in the spirit of the "weather stone": https://en.wikipedia.org/wiki/Weather_rock