Groco AS-100 Bilge Pump Switch 

"AS-100 uses the simple and reliable pressure fluctuation from a rising or falling bilge water level to turn your bilge pump on and off automatically. No floats to stick or fill with water. Approximate levels are on at 2-3/4" of bilge level and off at 1". Use with a pump that draws up to 10-amps running continuously or up to 20 amps running intermittently. NOT TO BE USED IN POTABLE WATER SYSTEMS."