Persons scuba diving, skin diving, snorkeling, or underwater spearfishing must display a diver-down flag to warn other boaters. A diver must stay within 150 feet of the flag. Unless there is an emergency, a diver may not surface more than 50 feet from the flag. It is unlawful to display a diver-down flag when not diving. Vessels not engaged in diving operations must stay 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. Divers also should follow all of the water safety rules themselves.
Two types of flags are used to indicate diving activity.
Divers Flag: A rectangular red flag with a white diagonal stripe that is one-fifth the width of the flag is required if on state waters. The flag must be at least 12 inches high and 15 inches wide and mounted to be clearly visible.
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.