I work in the Semiconductor Industry, so my perspective is a bit different. I've seen the WaterBug style used, as well as the Yamatake. The WaterBug uses electrical contacts feeding an OpAmp. Although we would consider it safe at 24DC, it still has a direct electrical contact to the water on the floor. The lack of isolation may be why it doesn't get UL508 approval?
Optical is the next choice if you don't want to sense it electrically. The Yamatake HPQ-D is typical of the different versions I've seen.
Yes, maybe a little high tech, because it's built for a harsh environment with solvents and acids. There are other manufacturers that make optical and ultrasonic sensors to detect liquid, but the Yamatake is the only one I've found that is designed to sit on the floor (bottom of the cabinet in Semiconductor applications).
Yamatake is a Japanese company. They may have UL508 approval, but just haven't updated the English version of their website yet. Give them a call or email. Tech support in both the US and Japan is good. Japan speaks (writes) English, and responds quickly.