Nobody is asking Metro to enforce or not enforce any federal law. Admittedly, I don't know enough about it to know what the applicable federal law is other than the one posted in that thread:
18 U.S. Code § 2277 - Explosives or dangerous weapons aboard vessels
(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 [...] shall be fined under this title or imprisoned not more than one year, or both.
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Presumably, whoever owns the vessels could grant permission to carry aboard the vessel and no particular law (that I know of) would be in question of being enforced or not enforced. If Metro owns them (I don't know if that's the case or not), they likely wouldn't grant said permission if asked.
Likewise, I don't know the terminology well enough to know what "the master of such vessel" means - whether that means the Captain behind the wheel, the owner, the person in charge of where it's tied up, something else, or some combination of the above. If it's the Captain, is it as simple as asking him for permission before boarding (though he may or may not be under direction from Metro)? Does anyone know who to contact in order to inquire about such permission? If the "master" has given permission and Metro (presuming that they are the owner) has not expressly forbade it (I see no mention of any policy whatsoever on the site), is that good enough even if asking the owner may result in a "no?"
Again, nobody is suggesting that Metro (or anyone else, for that matter) enforce or not enforce any law. I am asking on how to comply with the law.
ETA
Quoted:
[b]
I haven't heard any news from anyone lately on the subject of the Belle of Louisville, but I don't suspect that anything has changed. The last I heard about this, anyone found to be carrying a gun will be denied boarding on the Belle.
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Something has changed... the website used to explicitly forbid bringing weapons onboard, now it does not. It is most likely an oversight, but who knows. If the policy has changed and permission would be granted, thus allowing one to CCDW on board, some CCDW-ers may patronize the cruises who might not otherwise.