Federal law requires that scuba divers or snorkelers on federally controlled waters display the Alfa diver-down flag to mark the diving area. Vessels should stay as far away from a diver-down flag as is reasonable and prudent for the circumstances. The suggested safe distance from a flag is 100 yards.
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, at least 15 x 15 inches, with a white diagonal stripe is used on Mississippi state waters.
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.