I have no doubt,that Rounding the horn,was written by someone whoad a direct experience or who spent considerable time talking to sailors to get real experience.
lets take Ewan MaCColl,He was able to write convincing songs, because he had experience, he had sailors/fishermen talking about their experiences,he used the rhythyms of their speech,and their vernacular.
most broadside writers were concerned with making money as quickly as possible to sell a song,and getting a song out quickly.
so its possible that Rounding the Horn was the effort of a broadside writer but[imo]unlikely,on the other hand would a sailor have had the literary skills[many were illiterate].
of course it may have been folk processed,or it may be the work of more than one person.
I think it more likely to be the work of literate man who was at sea,perhaps a Captain,ships surgeon,or lower officer.
it is however a great song.