What must a towing vessel do to legally tow a person on water skis?

To legally tow a person on water skis, the towing vessel must have an observer on board. This requirement is in place to ensure safety while the skier is in the water. The observer's role is to focus on the skier, watching for any signs of distress or the need to stop the towing. This person can immediately communicate with the driver of the vessel to ensure that the skier is safe at all times.

Having an observer is crucial because it provides an extra set of eyes on the skier, which helps mitigate the risks associated with water skiing. The ability for the observer to signal to the operator enables a quicker response in case of any incidents or emergencies. Overall, this regulation enhances the safety protocol for towing skiers.

The other options do not fulfill the legal requirements for safe towing practices. For instance, while a skier-down flag is an important safety tool, it is not mandatory by law for the towing operation to be legal. Similarly, maintaining a speed over 30 mph is not a requirement, as safe towing can occur at various speeds depending on conditions and the skier's ability. Allowing the skier to detach at will is also a safety consideration but does not address the need for an observer on board during the towing activity.