BOTH INTERNATIONAL & INLAND By night, you sight the lights of a vessel engaged in underwater operations. If an obstruction exists on the port side of the vessel, how will it be marked?

Correct Answer: Option A — two red lights in a vertical line

Rule 27(d)(iii) specifies that a vessel engaged in underwater operations, when an obstruction exists, must display two red lights in a vertical line to indicate the side on which the obstruction exists. This warns approaching vessels to keep clear of the hazardous area.