When I called Carnival last year, before our cruise, they stated that it was against "International Maritime Law" to carry a firearm aboard a "civilian vessel." It did not matter if it was in a locked case or whatever and the (dare I say he sounded quite gay, simpers and lisps and all) guy on the phone was adamant that it was the "Maritime Law."
That response sounded fishy to me, so I did some research on my own. All I could find was reference to arming vessels, not with small arms but with the big stuff, nothing at all specifically about handguns etc. Nothing much on the web at all.
Several subsequent calls, never answered by a human, and emails to Carnival produced no reaction at all, not even a polite "don't bring a gun aboard" or anything.
When I arrived at the embarkation point, I carefully locked my gun in a case, in a locked "special spot" in my locked car and hoped that it would all be there when I got back. The lot I parked in was attended, at least minimally, and it was assigned parking for that cruise so it would be unusual for anyone to have any business being there while the ship was gone, so I felt pretty comfortable.
There were walk through metal detectors for boarding, but I don't have any idea if our baggage was x-rayed. I know my bags were not manually searched because my traps were not triggered.
As a funny aside, when we returned aboard in Cozumel, after a pleasant day of driving and diving, capped off by a little too much drinking, I set off the WTMD on my first pass. Reaching in my bathing suit pocket I found a can of Altoids, which I handed to the rent a cop and then I tried again. The second time through I realized I had forgotten this big silver bracelet I had on, and handed it to the same guy and tried a third time.
On my third time through, my ever helpful, but pretty sloshed, wife shouted for all to hear; "Honey, maybe it's your knife." Since we had been diving and I always carry a knife anyway, I had one clipped on my pocket that I had forgotten to take off.
But by this time the line behind me was getting long and the rent a cops were getting bored, so they handed me back my Altoids and bracelet and waved me on, even after my wife's comment.

Made me feel really secure, especially since I work for the TSA and only give people two chances before they get referred for special handling.