How many people must a vessel be rated to carry if an observer is on board while pulling a skier?

A vessel must be rated to carry at least three people when there is an observer on board while pulling a skier. This is because the law requires that there be a driver responsible for operating the boat, an observer to keep watch on the skier and ensure their safety, and the skier themselves. An observer is crucial for signaling and monitoring the skier, especially in cases where the skier may fall or require assistance. Thus, having at least three people ensures that each role is fulfilled adequately, promoting both safety and compliance with boating regulations.