Federal law requires that scuba divers or snorkelers on federally controlled waters display the Alfa diver-down flag to mark their diving area.
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 to indicate the presence of a submerged diver in the area. The diver flag must be displayed when a diver is in the water. Boaters should always use caution and slow to “slow-no wake speed” when operating within 200 feet of a displayed flag.
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 boats on federally controlled waters. This flag indicates that the boat is involved in a diving activity.