I wanted an alarm in the bilge to alert me of incoming water while out sailing. I ended up buying the $19 battery powered Sonin 03300 Wireless Water Alarm available from Amazon and have been happy with it. The main unit takes two double A’s, has no moving mechanical parts, has a 6 ft cable between the unit and sensor, and it has a test switch. It has an optional remote alarm as well (that I haven’t needed to use) that can either be powered by batteries or the included AC adapter.
The sensor is just two metal contacts that when connected via a conductor such as water sounds the alarm and it can easily be tested by dripping a little water on it. I didn’t even bother installing the remote receiver since I can hear the main unit’s alarm well enough even with the engine running.