I've gone cruising kilted before. No one knows what that little handle sticking out of your sock is, and no one really cares.

But what if they do care?

First things first, if you're behaving like a gentleman, no one will give much thought to you carrying a sgian dubh. An "armed" gentlemen is only a threat to rascals, reprobates and ne'er-do-wells, who tend not to cruise.

Second, if you're cornered on it, you can demonstrate how you're not a threat. Chances are, your sgian dubh blade is really dull. You can demonstrate that by pulling the blade across your palm and saying, "Look, this is just a ceremonial ornament, which I use to identify with my culture and heritage, which I take very seriously." (Of course, demo this for yourself, in private, before the cruise.)

Have fun on your cruise.