This affectation is not just in sea shanties though.
Steve, has it been established that this is an affectation of chanty singers? I've not seen it. And if I did, I'd suspect, as you say, that we are looking at an affectation of certain people of particular regions/dialects/etc of English, not a trait of the chanty genre.

Hugill seems to have bought into what I call the "sea exceptionalism" narrative. "Sailormen liked this sound, as can be seen from his pronunciation…" (Hugill). It's like talking about some exotic species, where every member of the class is essentially the same. ("The Eskimo has no concept of time.") When operating according to this narrative, instead of actually looking at the thing under study and assessing what you see, one picks out quirky things and amplifies them, to present to the "lay" audience and show how unique "sailors" are.