Description
A diver down flag, or scuba flag, is a flag used to indicate that there is a diver below. It is red with a white stripe from the upper left corner to the lower right corner. It can be placed on a boat or on a Surface Marker Buoy.
Purpose
It is used to notify to any boats to steer clear for the safety of the diver.
The use of this flag is required by law or regulation almost everywhere in the world. Usually the regulations require divers to display the flag and to stay within a specified distance of it when they are near the surface. As well there is often a larger zone around the flag where no boats are allowed to pass. Several states also prohibit the display of this flag when there is no diver in water.
Signal flag ALFA
The international Signal flag A (letter ALFA) is used as well to communicate the presence of a diver in water, but the meaning and the purpose are slightly different: it means that diving operations are restricting the ability of the ship to maneuver. As recent U.S. Coast Guard Notices to Mariners emphasize:
The ALFA flag is a navigational signal intended to protect the vessel from collision[1].