Working principle of echo sounder

An echo sounder is simply a device that is used to help sailors determine the depth of water at different locations in the sea. There is a transducer that is attached to the hull of the ship that produces sound waves directed at the seabed. Various sources say that the principle is simple: time taken for the sound waves to travel to the bottom and back is measured, speed of sound in the seawater is known, depth can thus be calculated. However, there are also some sources saying that the device actually makes use of the Doppler effect. I can see how this may come into play considering that the ship is moving as the sound waves move down and up again. However, I am unsure of how this works exactly. Can someone enlighten me on this?