I built an alarm out of an old smoke/fire alarm. So, when it sounds, it is the same as if a smoke or ionization detector goes off. Pretty hard to ignore unless you are a VERY HEAVY sleeper. As I remember, I ran two wires from either side of the actual smoke detector module to two wires that went to the basement floor just inside the drip pan. I bared about 4" at the end of each wire and placed them an inch apart on the pan. When water contacted both wires, the alarm went off just fine. Now the water wasn't totally clean as it picked up dirt as it dripped down the side of the heater, but it was enough to trigger the alarm. You could try doing the same thing with any old fire alarm.