RE: Harmony & exertion &c.

In maritime work song, only the chorus labours (burthen) and it's not always killer work. The verse belongs to middle managment but it's typically a solo in the Western form. Pacific-Asian styles have two or more song leaders so harmony can be a thing.

When mariners of diverse cultures come together in common song a kind of harmony comes naturally:

So two pilgrims who were priests and monks, and who had good voices, went along the rowing-benches as far as the mast, to the place where sea Mass is wont to be read, and there in union they began to sing with a loud voice the hymn of Ambrose and Augustine, (Te Deum laudamus) which was taken up by all the other clergy present as it is sung in church, each man singing it according to the notation of his own choir at home. I have never heard so sweet and joyous a song, for there were many voices, and their various dissonance made as it were sweet music and harmony; for all alike sang the same words, but the notes were different and yet sweetly harmonized together, and it was a joyous thing to hear so many priests singing the same song together out of the gladness of their hearts. There were many Latin priests, Sclavonians, Italians, Lombards, Gauls, Franks, Germans, Englishmen, Irishmen, Hungarians, Scots, Dacians, Bohemians, and Spaniards, and many there were who spoke the same tongue, but came from different dioceses, and belonged to different religious orders. All these sang the glorious Te Deum, in which even the laity, pilgrims, and the crew of the galley alike joined in, shouting aloud for joy at our good fortune. [Fabri, Howe Hissa &c.]