US Code, Title 14, Section 89:
Sec. 89. Law enforcement
(a) The Coast Guard may make inquiries, examinations, inspections,  searches, seizures, and arrests upon the high seas and waters over which  the United States has jurisdiction, for the prevention, detection, and  suppression of violations of laws of the United States. For such  purposes, commissioned, warrant, and petty officers may at any time go  on board of any vessel subject to the jurisdiction, or to the operation  of any law, of the United States, address inquiries to those on board,  examine the ship's documents and papers, and examine, inspect, and  search the vessel and use all necessary force to compel compliance. When  from such inquiries, examination, inspection, or search it appears that  a breach of the laws of the United States rendering a person liable to  arrest is being, or has been committed, by any person, such person shall  be arrested or, if escaping to shore, shall be immediately pursued and  arrested on shore, or other lawful and appropriate action shall be  taken; or, if it shall appear that a breach of the laws of the United  States has been committed so as to render such vessel, or the  merchandise, or any part thereof, on board of, or brought into the  United States by, such vessel, liable to forfeiture, or so as to render  such vessel liable to a fine or penalty and if necessary to secure such  fine or penalty, such vessel or such merchandise, or both, shall be  seized.
(b) The officers of the Coast Guard insofar as they are engaged,  pursuant to the authority contained in this section, in enforcing any  law of the United States shall:
(1) be deemed to be acting as agents of the particular
executive department or independent establishment charged with
the administration of the particular law; and
(2) be subject to all the rules and regulations promulgated by
such department or independent establishment with respect to the
enforcement of that law.
(c) The provisions of this section are in addition to any powers  conferred by law upon such officers, and not in limitation of any powers  conferred by law upon such officers, or any other officers of the  United States.