Name the principle used to find the depth of the sea (grade 9)

The principle of SONAR is used to find the depth of the sea. 

One method is Echosounding. A transducer (transmitter+receiver)  sends a sound pulse straight down into the water. The pulse moves down through the water and bounces off the ocean bottom. The transducer picks up the reflected sound. Computers precisely measure the time it takes for the sound pulse to reach the bottom and return. The depth of the ocean is then calculated by knowing how fast sound travels in the water (approximately 1,500 meters per second). 

Another similar method is the use of Fathomometer. It has an automatic drum which produces a sound, when the fathom line dropped reach the bottom, and it reaches the ears of the observers through a headphone.  Then the depth is computed on the basis of the fact that the sound waves travel through the water at the rate of 4840 feet (or approximately 1,500 meters) per second.