These are the marine flags signals you should know about.
Victorian law requires that any vessel with divers operating from it must always display signals by day and night to inform other vessel users.
The daytime signal for divers is an International Code Flag 'A' of size 6 being at least 750mm long and 600mm wide. It should be placed to ensure all-round visibility.
Divers operating without a vessel, for example from a jetty, should also display the International code flag "A". The flag should be at least 750mm long and 600mm wide. The flag may be displayed from a buoy. If displayed from a buoy, the flag must not be less than 300 mm in length and 200 mm in width. The flag must be clearly visible to all vessels operating in the vicinity.
During night diving, a vessel must show the international signal for a 'vessel restricted in its ability to manoeuvre'. These are three lights in a vertical line, top and bottom are red and the middle one is white. The diver's boat should also show other appropriate lights such as an anchor light.
People involved in night diving without a vessel should display a yellow (orange) flashing light that can be seen from a minimum distance of 200 metres.
In Victoria, Australia, a five knot (9.25 kph) speed limit applies to vessel operators and waterskiers within a distance of 100 metres (330 feet) of a vessel or buoy on which a diver below signal is displayed.
Other than the alpha flag, these are rarely seen by divers, however they may be useful to know.
The International Code Signal of Distress is indicated by the Code Flag "N" flying above the Code Flag "C".