Diver down flag
A diver down flag, or scuba flag, is a flag used on the water to indicate that there is a diver below. In North America it is conventionally red with a white stripe from the upper left corner to the lower right corner. Internationally, the code flag alfa/alpha, which is white and blue, is used to signal that the vessel has a diver down and other vessels should keep well clear at slow speed.
The purpose of the flags is to notify to any other boats to steer clear for the safety of the diver and to avert the possibility of a collision with the dive boat which may be unable to maneuver out of the way.
The use of the red and white flag, which was designed and introduced in 1956 by Navy veteran Denzel James Dockery, is required by law or regulation in many US states and Canada, as well as in several other countries in the world (e.g. Italy). 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. Some states also prohibit the display of this flag when there is no diver in water. It can be placed on a boat or on a surface marker buoy.
Signal flag ALFA/ALPHA 
As a code signal the International maritime signal flags A (letter ALFA/ALPHA) has the meaning of "I have a diver down; keep well clear at slow speed", used to indicate the presence of a diver in the water, and is more commonly employed in Europe and the British Commonwealth, including countries such as United Kingdom, Ireland, Australia, Canada, New Zealand, South Africa, and Kenya. It is also used by Russian Navy for the same purpose.
A rigid replica of the 'A' flag is required to be displayed by any vessel engaged in diving operations, when restricted in her ability to maneuver, if the size of the vessel makes it impractical to display the shapes and lights required by the International Rules for Prevention of Collisions at Sea (IRPCS) Rule 27.
The IRPCS rules are there to keep vessels clear of each other and in this way the A flag does protect divers as you are required to keep clear of vessels displaying flag A. Rule 27 leads to Rule 18 http://navruleshandbook.com/Rule18.html responsibilities between vessels and then rule 8 http://navruleshandbook.com/Rule8.html action to take. The rules also mention being seamanlike. Thus if a vessel shows flag A it is your responsibility to keep well clear in the understanding that divers are down, your action is to avoid collision and also act in a seamanlike manner. Seeing as the Flag A definition is International and defined in a strict legal framework if the rules are followed divers will be protected. The USCG definition muddies the waters in my view.
Although the presence of the 'A' flag may afford some protection for divers in the vicinity of the vessel displaying the flag, the intention of the rigid replica required by IRPCS Rule 27 (e) is to warn other vessels of the danger of collision. This marks a distinction between the 'A' flag and the red and white diver down flag.
Other uses 
Today the red and white flag is so strictly associated with scuba diving that it is also used to indicate a place where there are services for divers, for example stores selling or renting diving equipment or scuba service stations. It may be seen on the windows or bumpers of cars belonging to divers.
Code flag alfa/alpha also represents the letter 'A' in signalling.
- Cronje, Frans J (2007). "An Historic Overview of Recreational Scuba Diving.". In: Moon RE, Piantadosi CA, Camporesi EM (eds.). Dr. Peter Bennett Symposium Proceedings. Held May 1, 2004. Durham, N.C.: (Divers Alert Network): 6. Retrieved 2011-01-13.
- Richardson, Drew (1999). "A brief history of recreational diving in the United States.". South Pacific Underwater Medicine Society Journal 29 (3). Retrieved 2011-01-13.
- McMillan, Joseph (2001). "Maritime Warning Signals". Sea Flags. Retrieved 1 February 2010.
- "Italian regulations about the Diver Down Flag". D.P.R. n° 1639 del 2 ottobre 1968 (in Italian). Retrieved 1 February 2010.
- "U.S. Navy Signal Flags". United States Navy. 17 August 2009. Retrieved 1 February 2010.
- "Rule 27: Vessel not under command". U.S. Coast Guard Navigation Center. Retrieved 1 February 2010.
- "IRPCS Rule 27". Retrieved 1 February 2010.
- "Eighth Coast Guard District Special Notice to Mariners". United States Coast Guard. 2008. p. 23. Retrieved 2 February 2010. "The distinction the Coast Guard wants to make clear is: The Alpha flag is a navigational signal intended to protect the vessel from collision. The sports diver flag is an unofficial signal that, through custom, has come to be used to protect the diver in the water."
- (EN) Diver down flag history
- Wallbank, Alister (2001). "Can anybody see me? (modified reprint from DIVER 2000; 45 (2) February: 72-74)". Journal of the South Pacific Underwater Medicine Society 31 (2): 116–119. Retrieved 2008-10-13.