This simple project uses 555 timer IC in mono-stable mode. Initially the trigger is connected to the source via 10k resistor, and when the water level reaches the desired level, the trigger is partially connected to the ground thus making the output high. You can replace the LED with any circuit like relay to drive greater load like motors. You can build also a multiple stage water level indicator using transistor if you don't have any 555 at hand.
IC = NE555 Timer
Resistors = 10K, 20K, 1.5K
Capacitors = 0.001 or 0.01uf, 47uf