An instrument for determining the depth of water by measuring the time it takes a sound pulse to travel from the water's surface to the bottom and back, multiplying by the speed of sound, and dividing by 2. One type consists of an oscillator which generates the pulses, a hydrophone for detecting the echo, a clock for measuring the time elapsed between sending out the pulse and receiving the echo, and a recorder which converts the elapsed time to a corresponding depth and plots that depth on a continuous chart. Another type displays the depth digitally, records the value on magnetic tape as a function of location, and plots the depth on a fair sheet or chart. Early models of echo sounders used hammers or similar devices for generating pulses. Present day models use submerged loudspeakers or magnetostrictive or piezoelectric oscillators.