There isn't a much easier circuit to build if you want to make one. You need a battery (9 volt or less), a buzzer and some wire. However you mount these, and on a chunk of 2x4 is ok, the wire needs to run from the battery to the buzzer, then a gap of about an eighth of an inch in the wire and the gap needs to ultimately be placed where you expect the water to be, then the wire goes back to the battery. The water is what, in effect, closes the switch. When water bridges the gap, it completes the circuit and the buzzer goes off. Easy, eh? The "professional" models have better packaging but work the same.