"What I do find incongruous are chanties accompanied by guitar."

Well, I would think they are only congruous if they are being sung by seaman on deck for the appropriate tasks, or, possibly, by former seamen who did those tasks singing them for collectors. Once they become entertainment only, anything goes, as far as I'm concerned - as long as some outlandish claim of authenticity is not being made - but even then, that's showbiz, isn't it?