The Commander's Handbook on the Law of Naval Operations
Office of the Judge Advocate General, United States Navy
Chapter 2 - International Status and Navigation of Warships and Military Aircraft
[Relevant sections only. Particularly relevant sections highlighted in red.]
2.1 STATUS OF WARSHIPS
2.1.1 Warship Defined. International law defines a warship as a ship belonging to the armed forces of a nation bearing the external markings distinguishing the character and nationality of such ships, under the command of an officer duly commissioned by the government of that nation and whose name appears in the appropriate service list of officers, and manned by a crew which is under regular armed forces discipline. In the U.S. Navy, those ships designated "USS" are "warships" as defined by international law. U.S. Coast Guard vessels designated "USCGC" under the command of a commissioned officer are also "warships" under international law.
2.1.2 International Status. A warship enjoys sovereign immunity from interference by the authorities of nations other than the flag nation. Police and port authorities may board a warship only with the permission of the commanding officer. A warship cannot be required to consent to an onboard search or inspection, nor may it be required to fly the flag of the host nation. Although warships are required to comply with coastal nation traffic control, sewage, health, and quarantine restrictions instituted in conformance with the 1982 LOS Convention, a failure of compliance is subject only to diplomatic complaint or to coastal nation orders to leave its territorial sea immediately. Moreover, warships are immune from arrest and seizure, whether in national or international waters, are exempt from foreign taxes and regulation, and exercise exclusive control over all passengers and crew with regard to acts performed on board. . . .
2.1.3 Auxiliaries. Auxiliaries are vessels, other than warships, that are owned by or under the exclusive control of the armed forces. Because they are state owned or operated and used for the time being only on government noncommercial service, auxiliaries enjoy sovereign immunity. This means that, like warships, they are immune from arrest and search, whether in national or international waters. Like warships, they are exempt from foreign taxes and regulation, and exercise exclusive control over all passengers and crew with respect to acts performed on board.
U.S. auxiliaries include all vessels which comprise the Military Sealift Command (MSC) Force. The MSC Force includes: (1) United States Naval Ships (USNS) (i.e., U.S. owned vessels or those under bareboat charter, and assigned to MSC); (2) the National Defense Reserve Fleet (NDRF) and the Ready Reserve Force (RRF) (when activated and assigned to MSC); (3) privately owned vessels under time charter assigned to the Afloat Prepositioned Force (APF); and (4) those vessels chartered by MSC for a period of time or for a specific voyage or voyages. The United States claims full rights of sovereign immunity for all USNS, APF, NRDF and RRF vessels. As a matter of policy, however, the U.S. claims only freedom from arrest and taxation for those MSC Force time and voyage charters not included in the APF.
U.S. Navy and U.S. Coast Guard vessels which, except for the lack of a commissioned officer as commanding officer would be warships, also are auxiliaries.
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2.4.3 High Seas. All ships and aircraft, including warships and military aircraft, enjoy complete freedom of movement and operation on and over the high seas. For warships, this includes task force maneuvering, flight operations, military exercises, surveillance, intelligence gathering activities, and ordnance testing and firing. All nations also enjoy the right to lay submarine cables and pipelines on the bed of the high seas as well as on the continental shelf beyond the territorial sea, with coastal nation approval for the course of pipelines on the continental shelf. All of these activities must be conducted with due regard for the rights of other nations and the safe conduct and operation of other ships and aircraft.