Maybe just 2 wires, when there is moisture they will be shorted out some. Measure the voltage on one pin with analog pin with internal pullup enabled, and the other pin to arduino gnd.
Build some hysteresis into the system also. If the relay just switches on when the input level is above the threshold and off when the level is below the threshold, you will get a "jittering" effect on the relay as the input varies above and below this level. So switch the relay on when the input rises (or falls depending on how everything is wired) above an upper threshold and switch it off when it falls below a lower threshold.