an alarm system for a swimming pool or the like whereby small inexperienced children or animals are detected prior to entering the pool or after an unauthorized entry into the water of the pool . the invention comprises the use of a height sensing apparatus employing fibre optics and a logic circuit whereby an interruption of a pair of different elevations of light paths is accepted and the alarm remains silent and the interruption of the lowest light path only is reflected sounding an alarm . the invention further comprises the use of fibre optics and a movement sensor connected to the optics . any impact on the water surface of the pool causes the sensor to change the state of the light transmission therethrough by transmitting or interrupting light transmission . this change of normal state activates the alarm . the invention still further comprises a wireless means for detecting a person in the pool water or a man overboard from a ship or the like and sounding an alarm .