Originally posted by lasersailor184 
I don't really understand why it could be ruled either way.  However, I do know that the Coast Guard can stop any vessel on the water and perform a "Safety Inspection."  They do this to vessels they are pretty sure are performing illegal acts.
When, during this "Safety Inspection," they come across drugs or something like that, they then make an arrest for illegal activity.
How legal is this?  (honest question, not loaded at all) 
 Seeing as how I have been a Coast Guard boarding officer, I'll tell you.
14 USC 2 Established the Coast Guard
The Coast Guard shall enforce or assist in the enforcement of all applicable Federal laws on, under, and over the high seas and waters subject to the jurisdiction of the United States; shall engage in maritime air surveillance or interdiction to enforce or assist in the enforcement of the laws of the United States; shall administer laws and promulgate and enforce regulations for the promotion of safety of life and property on and under the high seas and waters subject to the jurisdiction of the United States covering all matters not specifically delegated by law to some other executive department; shall develop, establish, maintain, and operate, with due regard to the requirements of national defense, aids to maritime navigation, ice-breaking facilities, and rescue facilities for the promotion of safety on, under, and over the high seas and waters subject to the jurisdiction of the United States; shall, pursuant to international agreements, develop, establish, maintain, and operate icebreaking facilities on, under, and over waters other than the high seas and waters subject to the jurisdiction of the United States; shall engage in oceanographic research of the high seas and in waters subject to the jurisdiction of the United States; and shall maintain a state of readiness to function as a specialized service in the Navy in time of war, including the fulfillment of Maritime Defense Zone command responsibilities..[1] 
TITLE 14 - COAST GUARD
    PART I - REGULAR COAST GUARD
    CHAPTER 5 - FUNCTIONS AND POWERS
-HEAD-
    Sec. 89. Law enforcement
-STATUTE-
      (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.
So yes we CAN go onboard any US flagged vessel or any vessel in US waters anytime we want with NO probable cause and conduct searches, seize illegal contrband, and arrest violators all without a warrant.
Been there, done that, and have a couple dozen t-shirts.