I think it has to do with regulations. Guns/arms, equals a naval vessel, no guns equals a maritime vessel. IIRC most ports wont allow a naval vessel to dock without special permission. I heard about a cruise ship that is equipt with a sonic gun to ward off pirates, the greased sides is new to me.
I saw something on the news about 5 - 6 months ago. Some ships were greasing or oiling the sides of their boats to prevent the pirates from climbing the sides. Of course, I can't find it now - - but will keep looking.
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