On power-driven vessels, when must navigation lights be turned on?

The correct answer highlights that navigation lights must be turned on during periods of restricted visibility. This is essential for safety, as visibility can be reduced due to weather conditions such as fog, rain, or heavy darkness. Having navigation lights on allows other vessels to see your boat and helps prevent collisions.

While it is also important to have navigation lights on at night, the requirement during restricted visibility emphasizes the importance of being visible under various conditions, not limited solely to nighttime. The other scenarios, such as being moored at a dock, do not necessitate the use of navigation lights for safety reasons as the risk of collision is minimal in those situations. Hence, ensuring navigation lights are operational during restricted visibility is crucial for the protection of all waterway users.