It's the Law: Specifically for Skiing

Vessel operators towing a person(s) on water skis, aquaplane, kneeboard, tube, or a similar device have additional laws they must follow.

Requirements for Towing Skiers

Every vessel towing a person(s) on water skis or other devices must have on board, in addition to the operator, a person capable of observing the person(s) being towed and reporting their progress to the operator.
The observer must observe the person(s) under tow continuously and display a “skier-down” flag whenever the person is in the water after falling or while preparing to ski. The flag must be displayed so that it is visible from all directions.
Every person being towed behind a vessel on water skis or other devices must wear a USCG–approved life jacket (PFD). Inflatable life jackets are not recommended for persons being towed.
Everyone engaged in water-skiing—the operator, the observer, and the towed person(s)—must conduct themselves in a safe manner that does not endanger other persons or property.
It is illegal for vessels to tow a person(s) on water skis or any other device during the period from one hour after sunset until one hour before sunrise.
If towing a person on skis or other device, the vessel must be rated to carry at least the number of persons on board plus those being towed. You may not exceed the vessel's safe carrying capacity.