This is the official situation in the UK taken from
this website
Diving operations are indicated by flying one of two nationally recognized "Diver Down" flags, or navigation lights if at night. Either signifies a danger area. Stay well clear of diving operations because divers can easily stray from the diving vessel.
International Code Flag "A" has several uses, one of which is to designate diving operations. The Inland Rules specifically require a boat, when engaged in diving operations and thus restricted in its ability to maneuver, to display a rigid replica of the International Code Flag "A". Exhibiting this signal invokes a special right-of-way of this vessel over essentially all other vessels. The key point is
restricted in the ability to maneuver.
Code Flag "A" is used only on a vessel. It should never be displayed on a diver's float, or raft.
However, it is the practice of nationals of other countries to use the Code Flag "A" instead of the flag shown below. If the Code Flag "A" is seen on a diver's float or raft it should be treated as meaning:
Caution - Diver Down.
A red square flag with a diagonal white stripe is also an accepted and recognized signal meaning:
Caution - Diver Down. This flag is used on a vessel or on a diver's float, buoy, or raft to indicate diving operations. When divers are swimming free of the vessel, this would be the logical flag to use. The flag should be exhibited in rigid form.