When must lights be displayed on vessels?

The requirement for vessels to display lights is primarily intended for safety and navigational purposes, especially during times when visibility is limited. This means that lights must be utilized at night or in restricted visibility scenarios, such as fog, heavy rain, or other conditions that impair a mariner's ability to see other vessels. Displaying lights helps prevent collisions and enhances awareness of other vessels' positions, movements, and types.

In contrast, displaying lights only during daytime would not serve any safety function since natural daylight usually provides sufficient visibility for navigation. Bad weather conditions, while potentially impacting visibility, are too vague a criterion by themselves without a specific indication of restricted visibility. Additionally, the notion of requiring lights solely in harbors does not consider the navigational context outside of these areas, where safe navigation is crucial regardless of location. Therefore, the correct approach is to display lights at night or in restricted visibility to ensure overall maritime safety.