It's the Law: Specifically for Skiing

Vessel operators towing a person(s) on water skis or a similar device have additional laws.

Towing Skiers

A vessel towing a person(s) on water skis or any other similar device must have at least two persons on board—one to operate the vessel and a second person to observe the towed person(s). The observer must be at least 12 years old and be physically capable and mentally competent to act as an observer of a water-skier.
Each person being towed behind a vessel on water skis or a similar device must wear a life jacket (PFD) or buoyant belt. Note, however, that buoyant belts are not approved by the U.S. Coast Guard. The AZGFD recommends the use of a U.S. Coast Guard—approved, brightly colored Type III life jacket (PFD). The brightly colored life jacket (PFD) aids in making a skier more visible to other vessels.
Water-skiing is restricted to daylight hours only. It is illegal to tow a person(s) on water skis, surfboard, or other similar device from sunset to sunrise.
Both the operator of the towing vessel and the skier must operate in a safe manner. A reasonable distance from other vessels, people, and property must be maintained so as not to cause danger to the life or property of others.
State law makes the person being towed (the skier) responsible if he or she behaves in a careless, reckless, or negligent manner.
Vessels towing person(s) on water skis or a similar device must carry and use a bright red or orange skier-down flag that is at least 12 x 12 inches in size and mounted on a handle. The observer must continuously observe the skier(s) and display a skier-down flag whenever a skier is in the water after falling or while preparing to ski.
If towing a skier with a PWC, the same water-skiing laws apply. Do not exceed the carrying capacity of the personal watercraft.

Avoiding Propeller Strike Injuries

Most propeller strike accidents result from operator error. Victims include swimmers, scuba divers, fallen water-skiers, and boat operators or passengers. Most propeller accidents can be prevented by following basic safe boating practices.

Maintain a proper lookout. The primary cause of propeller strike accidents is operator inattention.
Make sure the engine is off so that the propeller is not rotating when passengers are boarding or leaving a boat.
Never start a boat with the engine in gear.
Slow down when approaching congested areas and anchorages. In congested areas, always be alert for swimmers and divers.
Learn to recognize warning buoys that mark swimming and other hazardous areas.
Keep the boat away from marked swimming and diving areas. Become familiar with the red and white or blue and white diver-down flags signaling that divers are below the surface.
Make sure that passengers are seated properly before getting underway. Some operators of larger boats with several passengers have caused injuries by putting the engine in gear while people were still swimming or diving from the boat.
Never ride on a seat back, gunwale, transom, or bow.