Final answer:
When approaching a power-driven vessel showing both red and green sidelights, both vessels should alter their course to the right to pass each other safely.
Step-by-step explanation:
When you sight another power-driven vessel dead ahead showing both the red and green sidelights, you are required to take immediate action to avoid a collision. In this case, since both the red and green sidelights are visible, it means that the other vessel is approaching head-on.
The International Regulations for Preventing Collisions at Sea (COLREGS) state that when two power-driven vessels are meeting head-on, both vessels should alter their course to starboard (right) in order to pass each other port (left) side to port side. This maneuver helps ensure a safe passing distance between the two vessels and reduces the risk of collision.
However, it's important to note that the specific actions to be taken may vary based on the exact circumstances and conditions at sea. Therefore, it's always recommended to follow the COLREGS and be prepared to take necessary evasive actions to prevent a collision. Additionally, it is a good practice to keep a radar plot to monitor the course and speed of the other vessel, as well as maintain a proper lookout using all available means such as visual and auditory cues.