My diving logbook is sitting out, waiting to be properly written up from a week or so ago.
It has two flags on the front.
The blue and white flag is the "A" or "Alpha" flag, internationally recognised from the signal alphabet. When a large A flag is flown from a vessel, it indicates that the vessel has divers in the water, and that other vessels should act accordingly. Usually that means keeping well clear, reducing speed, and keeping an extra watch for divers in the water.
The USA has of course chosen to go down a different route, as do territories close to or under the influence of the USA. There a vessel indicates divers below using a red flag with w white bar diagonally across it.
My diving folder, where I keep my qualification cards and log books, has both flags on the front, presumably to make it acceptable in all markets internationally...