The third person on board whose only job is keeping eyes on the rider, the rope and the board. In Germany they are legally required in addition to the driver.
At a cable park the installation and its staff run the operation; behind a boat that job belongs to a team of three roles — driver, observer and the rider on the board. The observer is not a post for whoever is bored: the driver is looking forward and simply cannot see you. The typical mistake is heading out as a pair and doing the role on the side. After every fall the same sequence applies: stop, turn, kill the engine completely before anyone climbs aboard — and never reverse toward a fallen rider, because that puts the propeller first. What tends to get missed: in Germany towed watersports are only permitted in daylight, from 8am at the earliest until 9pm at the latest, and every person being towed must be able to be taken aboard without exceeding the boat's permitted capacity.