Which piece of equipment is necessary for monitoring depth during a dive?

The necessary equipment for monitoring depth during a dive is a depth gauge. This instrument is specifically designed to measure the current depth of the diver underwater. It provides critical information that helps divers understand their position in the water column, which is essential for maintaining safety and managing exposure to pressure changes that occur with depth. Knowing how deep one is diving helps divers avoid potential hazards like decompression sickness and ensures that they adhere to their planned dive profiles.

In contrast, a numobarometer hose would not serve the purpose of monitoring depth as its function is related to measuring pressure in various applications, but not in a way that directly correlates to a diver's depth. Other options like a pressure regulator and an air tank are vital for supplying air to the diver but do not provide depth information.