During diving operations you must install a diver down flag. It is a flag coloured red with a white diagonal stripe, extending from the tip of the hoist to the bottom of the fly. It marks an area where scuba or other such diving activity is taking place.
If a diving operation takes place from aboard a pleasure craft, Code flag Alpha or flag A (white and blue) from the International Code of signals which means “I have a diver in the water, keep well clear and at low speed” must be displayed. While navigating, if you see this flag, you must keep well clear of the vessel and the diving site.