Types of Diver-Down Flags
Two types of flags are used to indicate diving activity.
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.
A blue and white International Code Flag A (or Alfa flag), at least 3.3 feet (one meter) high and visible from all directions. This flag must be displayed on vessels on federally controlled waters and indicates that the vessel is involved in a diving activity.
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.