Abstract: Ultrasonic directional guidance apparatus for use by divers underwater includes a pair of compact, self-contained units -- a pinger for generating omnidirectional signal bursts, and a unidirectionally sensitive hydrophone for use in locating the pinger. For economy of manufacture, the units ae structurally similar and have substantially identical watertight housings. A significant feature shared by the units is an easily operated switching arrangement for turning the units off and on, or for changing operating modes, i.e., hydrophone sensitivity or pinger signal transmission rate. The disclosed switching arrangement includes a collar carrying a magnet and mounted on the housing for rotation relative to it. Magnetic field-actuated switches inside the housing are selectively actuated to energize the units or change operating modes by rotating the collar to different detented positions, one for each switch, in which the magnet is sufficiently close for its field to actuate the switch.