What is required of an observer in a vessel towing a skier without a rearview mirror?

In a scenario where a vessel is towing a skier and does not have a rearview mirror, it is essential for safety that an observer is present on board the vessel. The role of the observer is crucial, as they are responsible for watching the skier at all times. This allows the observer to communicate with the driver, alerting them to any issues that may arise, such as the skier falling or needing to be picked up. Having an observer on board enhances overall safety, ensuring that both the skier and the vessel's operation are monitored effectively.

The other options do not align with the requirements for safe towing. For example, an observer on shore would not be able to assist or provide critical feedback to the driver of the boat. Using a personal flotation device is always a safety measure for any participant in water activities, but it does not fulfill the specific requirement of having an observer in the boat. Wearing a helmet is not a standard requirement for water skiing and does not pertain to the role of monitoring the skier during the towing process. Thus, the requirement for an observer to be on board stands out as the most appropriate answer to ensure safety while towing a skier.