What device is used to determine the distance of an object underwater by recording echoes of sound waves?

The device used to determine the distance of an object underwater by recording echoes of sound waves is sonar. Sonar, which stands for Sound Navigation and Ranging, operates by emitting sound pulses in water and then measuring the time it takes for the echoes of those pulses to return after reflecting off objects, such as the seabed or submerged objects. This timing provides the distance to the object based on the speed of sound in water.

In contrast, radar, which is primarily used for detecting objects in the air or on land, relies on radio waves rather than sound waves. An altimeter measures elevation above a surface, typically using atmospheric pressure, and a hydrometer is used to measure the specific gravity of liquids, which does not involve sound or distance measurement in the context of underwater objects. Thus, sonar is specifically designed for underwater applications, making it the correct choice for this question.