I had a water sensor (standalone, battery operated) that would detect the presence of water and sound the alarm. Something like that could be mounted on the side of the water level that you are measuring, and use it's state (water / no water) as a sensor and take action as needed. There's a number of water/leak detectors listed on Amazon that might do the trick if you can overcome the outside/inside limitations
BTW, thanks for the reminder, I intended to get one of these for my home!