Aquatic Telephones Let Skin Divers Talk Under Water, 1957

This swimmie-talkie uses water as a medium for sending high-frequency sound waves, on the principle of the hydrophone employed in the early 1900s for communicating between ships, and in World War I for detecting submarines.

Being adjusted here on a frogman, the Aquavox includes a face-mask mike, transducer (on belt, left), transceiver (right), earphones (on thigh). Cotton Associates, Philadelphia, developed it in 1957.