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The principal function of a fathometer is to make deep sea soundings. A fathometer, also known as a depth sounder, uses sonar technology to measure the depth of water beneath a vessel. It sends sound waves that travel through the water, reflect off the seabed, and return to the device. By calculating the time it takes for the sound waves to return, the fathometer can accurately determine the depth of the water. This measurement is essential for nautical navigation, understanding marine topography, and conducting underwater surveys.

The other choices, while relevant to oceanography, do not pertain to the specific function of a fathometer. For instance, measuring water temperature or salinity involves different instruments, and tracking marine life is typically accomplished using sonar or other specialized equipment designed to monitor fish and other aquatic organisms. Thus, the correct identification of the fathometer's function emphasizes its unique role in depth measurement.