Do you need repeatability or quick action? If neither, you could put a couple of screws through the jaws of a clothespin, attach your wires, then wedge them open with a small aspirin tablet. If it gets wet, the aspirin will dissolve, allowing the clothespin to close and complete the circuit. Another option might be to place a couple of wires near each other in a fine plastic mesh of some sort, filled with salt. If it gets wet enough, the resistance will change, and that could be detected. If you're trying to detect water level, a float switch like blueroom suggested is the best deal; if the water to be detected isn't distilled water (ie, it has impurities - most water does), then a simple PCB with a couple of closes spaced tracks etched next to each other will probably detect water by change in resistance...