Flags

The Alpha flag signifies restricted ability to maneuver and must be flown from the boat hosting divers or snorkelers.
This is the only flag that is required by federal law to be flown by boats mothering diver or snorkelers, and a rigid replica of it no less than 1 meter (3.3 feet) high must visible to other boats. At night, red over white over red 360 degree lights must be displayed.

The second flag, called the Diver Down flag, is probably more recognizable to most boaters. This flag is flown from a float in the water where people are engaged in diving activities.

It is not required by federal law to be displayed, however some states do require it. This flag will help other boaters know there are divers beneath the water who could immerge at any time.

Other boats that see these flags displayed should give the area a wide berth. Laws vary from state to state on how much distance a boater must give an Alpha flag or diver down flag; in some states it could be 300 feet, in others 100 feet. While boating anywhere near a diver down flag, keep a lookout for bubbles breaking the surface which could indicate divers who have strayed away from their boat.