What is the internationally-accepted method for warning ships about to enter restricted waters?

Question

In naval movies, when a warship encounters a boat entering restricted waters, the captain usually responds to this intrusion with the classic order "Fire a shot off their bow!".

In real life, what is the internationally-accepted method for warning ships of their intrusion into restricted waters before a warship can fire upon that boat?

Answer 1

as was mentioned, VHF Channel 16 or  SSB 2182kc;

international code flag "U" = "You are standing into danger" or sending "U" by flashing light

five short blasts on the ships horn repeated at short intervals

cutting back and forth in front of the ship in question

then the old shot across the bows.....

Answer 2

As I understand it, ships normally share many common radio frequencies, and it is a rare case when one ship cannot contact another and warn them via radio.

In modern times, the "shots off their bow" only happen after repeated attempts at radio contact as well as attempts to visually signal the other craft have failed, or if the offending craft is behaving in an aggressive manner, such as heading straight for a vessel.

Answer 3

VHF channel 16.