Boating flags / Marine flags are stocked for more than 100 different nations. International boat flags are also known as courtesy flags, The courtesy flag of the visited country replaces the yellow quarantine flag once cleared into port. International boat flags can also be used to display the heritage or nationality of the boat owner. Some international courtesy flags are the civil ensign of the nation, i.e. without the national coat of arms or emblem. Countries that specify the use of a civil ensign for marine use include Andorra, Argentina, Bolivia, Costa Rica, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Nicaragua, Peru, San Marino, Serbia, and Spain. Use of the national flag that includes the emblem or coat of arms can be viewed as either improper protocol, or in some cases, illegal.

Information for Display of Courtesy Flags & Foreign Guest Flags (ref; United States Power Squadron)
If your vessel is mastless, it should display the courtesy flag at the bow, in lieu of a squadron or club burgee, or on a starboard antenna strong enough to support it. If your vessel has one or more masts, display it single-hoisted at the outboard signal halyard of the main starboard spreader. Move any flag normally flown there to the inboard starboard halyard or, if your boat has only one halyard per side, to the port spreader halyard.

The customs observed in various foreign waters differ from one another. It is best to learn the correct procedure for the country you are entering. In some countries it is customary to fly the courtesy flag only after the quarantine flag (the yellow 'Q' flag) and the vessel has been granted pratique by the appropriate authorities.

Do not fly a foreign courtesy flag after you have returned to U.S. waters. Do not display U.S. yacht ensigns in foreign waters.

When a foreign guest is aboard, the ensign of the guest's country may be displayed from the bow staff or outboard port spreader. Should more than one such guest flag be appropriate, wear them on spreader halyards from port to starboard in the alphabetical order of their countries' names in the English language

Pratique - def. "License or permission to use a port, given to a ship after quarantine or on showing a clean bill of health".

When an American boat crosses the national boundaries of the United States to visit a foreign port, or a foreign-yacht visits an American port, certain customs, immigration, and other regulations must be obeyed. As a result of various provisions and exemptions applying to yachts, not to vessels engaged in trade, the procedure has been so simplified that there is nothing in these legal requirements to deter a recreational craft from enjoying a cruise outside U.S. waters. There are severe penalties, however, for failure to observe the regulations that do apply.

On arrival at a foreign port, the owner or captain of a yacht (any size) should report to the authorities. The "Q" flag (plain yellow) should be flown where it can be easily seen-starboard spreader, radio antenna, fishing outrigger, etc.- to indicate that the vessel desires to be boarded by customs and other governmental authorities. When reporting, the crew and guests must remain aboard until permission (pratique) has been granted to land. Once pratique has been granted, the "Q" flag may be lowered and the courtesy flag of that nation may be hoisted. Traditionally, quarantine flags are not used for trips between the United States and Canada on the Great Lakes.