They make a commercially available pool alarm, battery powered with enclosed tilt switch and piezo buzzer, it floats freely in the pool. When the water is disturbed the waves activate the tilt(pendulum?) switch which turns on the piezo thru a scr which has to be manually reset. The one I took apart also had a load resistor across the piezo to keep the scr turned on since the piezo draws very little current..The reset switch was a normally closed reed switch that required placing a magnet nrxt to it, this was done to keep it enclosed and water tight I suppose..