What switch is used to turn on the fathometer depth sounder?

The power switch is the correct choice for turning on the fathometer depth sounder because it is the primary control that supplies electrical power to the device. Fathometers, also known as depth sounders, require a powered state to operate effectively, as they rely on signals transmitted and received to measure water depth.

While options like the main switch, depth switch, or control switch might seem plausible, they typically refer to different functionalities or systems. The main switch might operate overall power systems, while a control switch would likely manage different aspects of operations, and a depth switch may adjust settings related to the depth measurement but would not initiate the power necessary for the device to function. Therefore, the power switch is specifically designated for turning devices on and off, making it the essential choice for activating the fathometer.