Which of the following tools is NOT useful for deep dives?

In the context of deep dives, each tool has its specific uses related to safety, navigation, and communication. The underwater camera, while a valuable piece of equipment for capturing images and documenting marine life, is not essential for deep diving. It does not play a direct role in enhancing safety or facilitating navigation at depths where visibility can be limited and where divers may face challenges such as narcosis or high pressure.

On the other hand, a reference line is crucial for maintaining a safe ascent and descent profile, emergency oxygen is vital for managing potential dive-related emergencies, and a dive light is important for illuminating the underwater environment in low-light conditions often found at greater depths. Therefore, the underwater camera stands out as the tool that is least necessary for ensuring safety and effectiveness in deep dives.