19 CFR 162.3 - Boarding and search of vessels.
(a) General authority. A Customs officer, for the purpose of examining the manifest and other documents and papers and examining, inspecting and searching the vessel, may at any time go on board:
(1) Any vessel at any place in the United States or within the Customs waters of the United States;
(2) Any American vessel on the high seas;
(3) Any vessel within a Customs-enforcement area designated such under the provisions of the Anti-Smuggling Act (Act of August 5, 1935, as amended, 49 Stat. 517; 19 U.S.C. 1701, 1703 through 1711), but Customs officers shall not board a foreign vessel upon the high seas in contravention of any treaty with a foreign government, or in the absence of a special arrangement with the foreign government concerned.
(b) Search of army or navy vessel. If the port director or special agent in charge believes that sufficient grounds exist to justify a search of any army or navy vessel, the facts shall be reported to the commanding officer or master of the vessel with a request that he cause a full search to be made, and advise the port director or special agent in charge of the result of such search. If, after the cargo has been discharged, passengers and their baggage landed, and the baggage of officers and crewmembers examined and passed, the port director or special agent in charge believes that sufficient grounds exist to justify the continuance of Customs supervision of the vessel, the commanding officer or master of the vessel shall be advised accordingly.
(c) Assistance of other agencies. Customs officers are authorized to assist any other agency in the enforcement of United States laws on any vessel.