Diver down flag

A flag, hoist half white, fly half blue, the fly edge has a triangular swallowtail cut-out along its full length
A red flag with white diagonal band
The "Alpha" or "Alfa" flag (left) and red-and-white flag (right), both meaning "I have a diver down; keep well clear at slow speed"

A diver down flag, or scuba flag, is a flag used on the water to indicate that there is a diver below. Two styles of flag are in use. 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. In North America it is conventionally red with a white stripe from the upper left corner to the lower right corner.