A quick measurement here shows that two wires dipped into the local water produces about a 1M resistance. Adding salt brings it down to about 400K.
Even without the salt, using a voltage divider I could get reliable detection of dipping the wires into water:
void setup ()
pinMode (13, OUTPUT);
void loop ()
digitalWrite (13, digitalRead (2));
I put a 3.9 M resistor between pin D2 and Gnd, and connected the sensor wires thus:
Theoretically that should give 4V on D2, and that is enough to turn it on. Or you could use an analog port for perhaps more sensitive readings.