The echo sounder is used on ships to find the position of the sea bed, to detect sunken wrecks or submarines and to find shoals of fish.
A transmitter at the bottom of the ship sends out a beam of sound waves that is reflected from the sea bed and then picked up by a receiver on the ship.
If you know the speed of sound in water and how long it is between transmitting a pulse of sound and
receiving back at the ship you can work out how deep the sea is at that point.
Depth of sea = [Speed of sound x Time]/2
where the time in the formula is how long it takes the sound to go to the sea bed and back.
There is a distinct difference between a sound pulse reflected from the sea bed and one reflected from a shoal of fish. The one reflected from the sea bed is distinct and not spread out in time while the one from the fish is more diffuse (woolly) and spread out as it reflects from fish at slightly different depths.
Moving objects like submarines and fish make the pitch of the reflected sound alter — this is called the Doppler Shift: