Persons scuba diving, skin diving, and snorkeling must display a diver-down flag to mark their diving area. Between sunset and sunrise, the flag must be lighted. Vessels not engaged in diving operations must remain at least 150 feet from a displayed diver-down flag.
Scuba divers and snorkelers should not place a flag in an area already occupied by other boaters or where their diving operation will impede the normal flow of waterway traffic.
Two types of flags are used to indicate diving activity.
Divers Flag: A rectangular red flag with a white diagonal stripe floats in the area of divers.
Alfa Flag: A blue-and-white International Code Flag A (or Alfa flag), at least 3.3 feet (one meter) high and visible from all directions, must be displayed on vessels on federally controlled waters. This flag indicates that the vessel is involved in a diving activity.