Admiralty law or maritime law is the distinct body of law (both substantive and procedural) governing navigation and shipping. Topics associated with this field in legal reference works may include: shipping; navigation; waters; commerce; seamen; towage; wharves, piers, and docks; insurance; maritime liens; canals; and recreation. Piracy (ship hijacking) is also an aspect of admiralty.
The courts and Congress seek to create a uniform body of admiralty law both nationally and internationally in order to facilitate commerce. The federal courts derive their exclusive jurisdiction over this field from the Judiciary Act of 1789 and from Article III, § 2 of the U.S. Constitution. Congress regulates admiralty partially through the Commerce Clause. American admiralty law formerly applied only to American tidal waters. It now extends to any waters navigable within the United States for interstate or foreign commerce. In such waters admiralty jurisdiction includes maritime matters not involving interstate commerce, including recreational boating.
Under admiralty, the ship's flag determines the source of law. For example, a ship flying the American flag in the Persian Gulf would be subject to American admiralty law; and a ship flying a French flag in American waters will be subject to French admiralty law. This also applies to criminal law governing the ship's crew. But the ship flying the flag must have substantive contacts with the nation of its flag in order for the law of the flag to apply. American courts may refuse jurisdiction where it would involve applying the law of another country, although in general international law does seek uniformity in admiralty law.