If a sailor needs to see the current depth, which system feature should be referenced?

The correct choice is to reference sonar when a sailor needs to see the current depth. Sonar, which stands for Sound Navigation and Ranging, operates by emitting sound pulses into the water and measuring the time it takes for them to bounce back from the seabed. This technology effectively provides real-time information about the water's depth beneath a vessel, allowing sailors to navigate safely and avoid underwater hazards.

In contrast, while a speedometer measures the speed of the vessel through the water, it does not provide any information about depth. GPS is primarily used for determining a vessel's position on the surface of the Earth and does not interact with water depth measurements. Charts can indicate depth in a broader sense, showing contour lines and soundings, but they do not provide real-time data, which is essential for immediate navigation needs. Thus, sonar is the most appropriate system for obtaining current depth information while at sea.