From OpenwaterpediaI have a diver down; keep well clear at slow speed.
U.S. Coast Guard regulates its use in Rule 27e (Vessel Engaged in Diving Operations), "Whenever the size of a vessel engaged in diving operations makes it impracticable to exhibit all lights and shapes prescribed in this Rule, the following shall be exhibited:
Three all-round lights in a vertical line where they can best be seen. The highest and lowest of these lights shall be red and the middle light shall be white; a rigid replica of the International Code flag "A" not less than 1 meter in height. Measures shall be taken to ensure its all-round visibility."
The alpha flag is established and recognized internationally for divers who are mostly below the surface of the water.