Admiralty is the body of private and public law dealing with ships and those matters dealing with ships, such as ship's personnel, ports, the high seas, navigation and the financing of ships. It is one of the earliest forms of international law and largely exists to establish a common body of law and practice between nations regarding these matters. It is one of the reasons many aspect of shipping are identical in almost every country, such as the rule that vessels under power must always yield to vessels under sail.
Many jurisdictions have special Admiralty courts. In jurisdictions where there is a lot of shipping, the practice of many lawyers is often dominated by Admiralty law.
The basic rule of Admiralty is that the law that applies onboard any particular ship, both criminal law and civil law, is determined by the ship's "flag", or the country where it is currently registered. Ships are required to fly their flag of registration to clearly indicate this fact. They are also required to clearly display their port of registration on the rear of the ship, as well as the ship's name. This information indicates where the ship's records are kept, so that the ship's title can be checked to determine it's owners, as well as other records, such as any security interest against the ship.
As such, the laws of local ports where the ship might be may have limited application to that ship. For example, a ship registered in the United States that is used in the United States for a criminal purpose may be subject to forfeiture to the government. However, if that ship is registered elsewhere, the law of the jurisdiction where the ship is registered applies because a U.S. court has no jurisdiction over a ship registry in another country. Similarly, competing claims against the ship generally must be settled in the manner they would be settled in the ship's home port rather than the port where the ship is presently held.