I concur with Gibb that a shanty is well defined as a "framework," i.e. a melody and a solo/chorus pattern. I'm strongly tempted to add to that at least one "regulation" solo line to confirm the song's identity, but I won't insist that it's necessary - yet.

To take an imaginary counterexample: if a shantyman began singing the words of the ballad "My Son David" in the framework of "Highland Laddie," with no overlapping filler verses, I think we'd still say it was a different shanty on the basis of the lyrics.

I thought of "My Son David" because the tune Jeannie Robertson used in the renditions that made her famous really does resemble "Highland Laddie"!