The best sensor technology for a water level threshold indicator uses a water/air defraction LED emitter detector pair. I don't have a part number with me.

You don't really require a micro unless you want to include datalogging of events.

I'd recommend two sensors, Max Level and Min Level.

Depending upon your application, whether filling or emptying a sump, one would control the motor-on state transistion and the other the motor-off transition. The spacing between the sensors give you hysterisis.

aka J