Final answer:
The sighting of three red lights in a vertical line on a vessel at night indicates a mine-clearing operation. The vessel displaying such lights is a mine-clearance vessel and should not be approached.
Step-by-step explanation:
If you observed three red lights in a vertical line on another vessel at night, it would be an indication that the vessel is a mine-clearing vessel. According to international maritime signal lights configurations, such a display signifies that the vessel has explosive charges in the water, is engaged in mine-clearance operations, and that it is at a standstill or not under command. Such vessels require a wide berth and should not be approached.
A vessel displaying three red lights in a vertical line dead ahead at night is likely a vessel being towed. According to the International Regulations for Preventing Collisions at Sea (COLREGs), a power-driven vessel towing astern (behind) will exhibit this configuration of lights. The three vertical red lights indicate the length of the tow exceeding 200 meters.
In this arrangement, the highest and lowest lights are red, and the middle one is also red but of greater intensity. The towing vessel itself will exhibit the normal navigation lights appropriate to its size and type. The purpose of these specific towing lights is to alert other vessels to the presence of a tow with an extended length, helping mariners to assess the towing vessel's size and the overall length of the tow for navigational safety and collision avoidance at sea.