For indicating the absence of a drain plug from a boat drain when the boat is in a launch condition, a receptacle with a water flow passage and a cavity containing a Hall effect switch is mounted in the boat drain. A stopper has a magnet which operatively aligns with the Hall effect switch when the stopper plugs the passage. The Hall effect switch and a switch operated in response to a presence-of-water condition are components of a control circuit which drives a switch to connect and disconnect an audio alarm activation device to and from an electrical power source. The alarm will always be activated when the stopper is not in the drain and the presence-of-water switch has operated and will never be activated when the stopper is in the drain.