Final answer:
A rearview mirror is needed when a vessel is towing a skier without a capable observer on board to monitor the skier's safety. This allows the driver to keep an eye on the skier, which is a safety measure required by law or regulations in many areas.
Step-by-step explanation:
A rearview mirror is required when a vessel is towing a skier and does not have a capable observer on board in addition to the driver. In water sports, it's essential to maintain safety for all participants, including the skier being towed and other individuals in the water. A rearview mirror on the boat allows the driver to keep an eye on the skier, ensuring they can respond promptly to any potential issues or if the skier falls into the water.
Many jurisdictions require the presence of an observer aside from the driver to monitor the skier's safety. However, if an observer is not present, a wide-angle rearview mirror that provides a clear view of the skier at all times can serve as a substitute. This requirement is set to enhance the safety measures during water sporting activities and is mandated by law or safety regulations.