I'll put what is happening and I'll let you put it into your own words.
Point A is emitting a sound wave that travels down into the water and finds the ship (point B). The sound wave then bounces off the the ship and travels to a receiver that is located on the back of the ship (Point C). The ship then calculates the time from the emission of the wave to the reception of the wave and from there calculates the depth of the object.
