A quick Google search turned up International Nautical Flag Codes and Ship Signals.
The one I see is what's shown as "DD 1". The "Alpha" on that page is not what you describe, since it's rectagular.
I also see Signal Flags that shows the blue and white one as "A". That's sometimes used in other countries to mean "Diver down", but it's real meaning is actually "reduced maneuverability", such as an anchored dive boat. My NAUI Worldwide textbook says this is used coloquilly as Diver Down, but really you should fly both flags.
—John
The nice thing about standards is there are so many to choose from.
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