One easy way may be to get your state's boating regulations or boat operators handbook. Most of these publications have a section on a boaters responsibility to avoid and a diver's responsibility to display the divers down or alpha flag.
the minnesota boaters handbooks states
SCUBA DIVING
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[FONT=Optima LT Std,Optima LT Std]Scuba divers must [/FONT][/FONT]
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[FONT=Optima LT Std,Optima LT Std]display a warning
flag when diving.
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[FONT=Optima LT Std,Optima LT Std]The diver’s flag must measure at least 15 inches horizontally and 12 inches vertically. [/FONT][/FONT]
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[FONT=Optima LT Std,Optima LT Std]Both sides must have a red-colored back- ground bisected diagonally by a three-inch
wide white stripe. There is also a blue and white diver’s flag authorized under the
federal rules of the road.
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[FONT=Optima LT Std,Optima LT Std]No more than four divers shall dive under one flag. [/FONT][/FONT]
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[FONT=Optima LT Std,Optima LT Std]Divers must remain within 50 feet (measured horizontally) of the warning flag. [/FONT][/FONT]
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[FONT=Optima LT Std,Optima LT Std]If a group of divers is using a contained area, the perimeter of the area must be marked. The area must be outside the normal area [/FONT][/FONT]
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[FONT=Optima LT Std,Optima LT Std]of navigation. These markings must consist of
the official diver’s flag and must be placed
around the perimeter of the diving area at
intervals of not more than 150 feet.
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[/FONT][/FONT]Do not place a diver’s flag where it will ob- struct navigation.
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[/FONT][/FONT]The diver’s flag may be displayed on a water- craft or float or be anchored to the bottom. The top of the flag must be at least 30 inches above the surface, however.
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[/FONT][/FONT]Boats not involved with the diving operation must remain 150 feet away from a flag.
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[/FONT][/FONT]Persons who dive at any time from sunset to sunrise must carry a diver’s light visible when above the water for a distance of 150 feet.
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[/FONT][/FONT]Scuba or skin diving while in possession of a spear is unlawful from sunset to sunrise.
http://files.dnr.state.mn.us/rlp/regulations/boatwater/boatingguide.pdf
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