An echo sounder is a device used to measure the depth of water by sending sound waves into the water and recording the time it takes for the echoes to return. This technology is commonly used in marine navigation and fishing to identify underwater features and locate schools of fish. The device operates by emitting a sound pulse, which travels through the water until it hits the seabed or an object. The time taken for the echo to return is then calculated to determine the depth. Echo sounders are essential tools for vessels like fishing boats and research ships.