If the ferry is a documented vessel (as most commercial vessels are that transport passengers for hire) then you might want to reconsider the previous advice.
TITLE 18 - CRIMES AND CRIMINAL PROCEDURE
PART I - CRIMES
CHAPTER 111 - SHIPPING
-HEAD-
Sec. 2277. Explosives or dangerous weapons aboard vessels
-STATUTE-
(a) Whoever brings, carries, or possesses any dangerous weapon,
instrument, or device, or any dynamite, nitroglycerin, or other
explosive article or compound on board of any vessel documented
under the laws of the United States, or any vessel purchased,
requisitioned, chartered, or taken over by the United States
pursuant to the provisions of Act June 6, 1941, ch. 174, 55 Stat.
242, as amended, without previously obtaining the permission of the
owner or the master of such vessel; or
Whoever brings, carries, or possesses any such weapon or
explosive on board of any vessel in the possession and under the
control of the United States or which has been seized and forfeited
by the United States or upon which a guard has been placed by the
United States pursuant to the provisions of section 191 of Title
50, without previously obtaining the permission of the captain of
the port in which such vessel is located, shall be fined under this
title or imprisoned not more than one year, or both.
(b) This section shall not apply to the personnel of the Armed
Forces of the United States or to officers or employees of the
United States or of a State or of a political subdivision thereof,
while acting in the performance of their duties, who are authorized
by law or by rules or regulations to own or possess any such weapon
or explosive.
-SOURCE-
(June 25, 1948, ch. 645, 62 Stat. 804; Pub. L. 103-322, title
XXXIII, Sec. 330016(1)(H), Sept. 13, 1994, 108 Stat. 2147; Pub. L.
109-304, Sec. 17(d)(6), Oct. 6, 2006, 120 Stat. 1707.)
