This circuit implements a simple water level detector and a gated oscillators to create a pulsed tone to drive the piezoelectric-bell audible alarm
http://www.e-insite.net/ednmag/index.asp?layout=article&stt=000&articleid=CA253786&pubdate=10/31/2002&spacedesc=designideas
controls two electric water pumps.
http://www.globaldialog.com/~jrice/diy/wavemaker.htm
http://www.citycom.gr/electronics/downcircuits/29.zip
http://www.citycom.gr/electronics/downcircuits/66.zip
sounds a beeper when its electrodes detect water.
http://www.imagineeringezine.com/PDF-FILES/h2oalm.pdf
circuit is designed to display 12 different levels
http://www.electronicsforu.com/efylinux/circuit/cir105.htm
http://www.digital-avatar.com/members/et/projects/water_level_sensor.htm
http://www.electronicsforu.com/electronicsforu/Lab/freecircuitslist.asp?id=48