[Lab] Water sensor for an arduino?

[Justin]

With spring comes flooding.

I have a pump, but it doesn't have an auto shutoff, and it shouldn't run dry.

I have a powertail that I can use to turn the pump on and off, so now all I 
need is a water sensor.

Would this be as simple as two nails in a board with wires attached to the 
nails? Water is conductive, when the water completes the circuit, the 
Arduino notices and turns on the pump.

Probably two water sensors, with the "on" sensor being above the "off" 
sensor to prevent it from triggering on and off too fast.

Is anything wrong with my logic? Why won't this work? Do I need more than 
wires to make a water level sensor?