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.