Use a plain transistor
You could build a simple set of moisture detectors to find out the water level. Water conducts electricity, albeit poorly, but enough to be detected and amplified by a simple transistor.
Any simple NPN transistor will work, from 2N2222, 2N3904, etc.. The relay should have a 500 Ohm coil or so. This circuit will close the relay when the moisture level exceeds a level set by the 1M variable resistor.
The probes can be bare metal or wires.
You can modify this to switch on higher powered devices like the pump or use more sensors in a column with a transistor array. I personally think you'd be better off with a mechanical solution...far safer.
Hope this helped!