Abstract: A swimming pool alarm has a hydrophone located well below the surface of the pool and spaced from its walls, the hydrophone being associated with a captive sensor element which responds to vertical wave motion of the pool water at levels well below the surface by impacting the hydrophone transducer. The hydrophone is associated with an amplifier and peak detector, elevation of the detector output above a set threshold triggering an alarm either responsive to high level audio signals reaching the hydrophone, or to deep water disturbance within the pool.