The potential problem with this method is the purity of the water involved. If it is relatively pure, it won’t conduct when it hits (or in this case when it leaves) the sensor and you may not get an alarm. You would probably be better with a floating type switch like the ones listed here (although there still isn’t a Fritzing part that I’m aware of for one)
The float only depends on the water level dropping, not the purity of the water involved.
Peter