you should use 2 contacts. apply a square wave on one and try to detect the signal on the other. use an op-amp and adjust the gain for the sensitivitiy. also, puse distilated water doesn't conduct electricity, but if it contains minerals and other stuff the conductivity varies a lot.
you should not use a continous courent for the testing probes, because of the electrolize. the coutacts will oxidate and you will find out that the circuit will not work well. i have a simple circuit that is it used just to detect water between the contacts. i'll check if it can be modified.