Most people have probably never heard of it, let alone know what it looks like. A diver down flag (or skin diver flag), is a flag that is used on the water to indicate that there is a diver below. The diver down flag is a bright red flag with a diagonal white stripe. The colors serve as an alert signal to anyone in the surrounding area, telling them to stay away from the immediate area around the diver down flag. Nearby boats are supposed to stay anywhere from 50 and 300 feet away from the diver down flag.
If the divers are not diving from a boat, the diver down flag needs to be displayed on a flotation device that is anchored within a few hundred feet of the divers. The diver down flag is used in conjunction with an alpha flag when diving from a boat.
The alpha flag is a white and blue flag with a triangular notch on the free end. This flag should be flown by a boat whenever the mobility of the boat is hindered. Many times, divers will display this flag when they leave the boat because it signals to nearby vessels that the boat will have difficulty moving quickly, and that nearby vessels should yield to the boat displaying the alpha flag.
The purpose of both flags is to notify nearby boats to steer away from the flags in order to protect the safety of the divers below. Divers cannot see what is above the water when they are underwater, so both of these flags are meant to protect divers from boating injuries. Both alpha, and dive flags need to be displayed whenever there is nearby boat traffic. However, it is best practice to display both flags whether you expect boat traffic or not.