State law requires that scuba divers or snorkelers display a diver-down flag to mark the diving area. Divers and snorkelers must stay within 75 feet of a diver-down flag at all times. Vessel operators must stay at least 150 feet away from a displayed diver-down flag. If there are circumstances that prevent the vessel from staying 150 feet way from the diver-down flag, the operator must throttle down to headway speed.
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 white diagonal stripe used to indicate when diving activities are in progress on New Hampshire 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.