A diver-down flag warns boat operators that a scuba diver is in the water and they should proceed slowly, keeping a wide berth from the divers. The onus is on the divers to stay within a specific distance of the flag when near the surface, and also underwater. It is within a larger area around the flag that boats may not enter, usually between 50 to 300 feet of the dive flag. This flag is also used to identify a location where diving gear is sold or rented.
The flag is used in North America, Italy and some other nations. It is red with a white stripe diagonally across from the upper left corner to the lower right.
The international Alpha flag is blue and white and is flown by dive boats when their manoeuvring is limited. Its purpose is to warn other boat operators of the danger of collision.
In the case of a diver drifting away from the location of the flag, it is highly recommended (and the law in some areas) to carry a inflatable surface marker buoy (SMB). Attach the buoy to a reel, inflate it and send it to the surface before attempting to exit from below.
All dive flags should be stiff enough to remain visible without wind.
Not all boaters are familiar with the use of dive flags, or even notice their presence. Before surfacing, divers should always look, and listen to make sure that no boats are in the area where they wish to surface.
As a diver it is your reasonability, to familiarize yourself with local diving regulations concerning dive flags especially in a new diving location.