Persons participating in scuba diving, skin diving, snorkeling, or underwater spearfishing must display a diver-down flag unless they are within 150 feet of shore. Vessels not engaged in diving operations must stay at least 100 feet away from any 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 three-inch white diagonal stripe. The flag must be at least 12 inches high and 15 inches long, and mounted on a float or buoy 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.