After its creation in the 1950s, the Diver Down flag quickly became the most common flag used by divers in the United States. However, it’s not part of the international code of signal flags, and many boats end up flying two flags when they have divers in the water—both the red and white Diver Down, and the traditional Alpha flag.
The core purpose of these flags is to let boats know that there is a diver in the water. When flown, both boats and divers have responsibilities: passing boats must not come within a certain distance of the Diver Down flag, and divers must remain close to it.