Persons scuba diving, skin diving, or snorkeling must display a diver-down flag to mark their diving area. Vessels not engaged in diving operations must stay at least 50 feet away from a displayed diver-down flag. Two types of flags are used to indicate diving activity.
A rectangular red flag with a white diagonal stripe can be attached to a vessel, float, or buoy.
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.