A free-floating self-contained swimming pool alarm unit in which a hollow housing forms a float structure having an internal chamber within which an audible alarm device is mounted adjacent sound transmitting openings in a top wall of the housing. An upstanding flared chimney surrounds the openings and provides a sound amplifying horn, the outer end of the chimney being provided with a shielding cap. Electronic control is arranged to trigger and activate the alarm device by connecting it to an electric source in the housing in response to the operation of a liquid sensor switch as a result of tilting movements of the housing due to wave disturbances produced by an object or person falling into the pool. Once the alarm is triggered, it will continue to operate until deactivated by an appropriate switch such as a manual switch mounted in an accessible portion of the housing.