The reason to sing harmonies is to bring overtones to the vocal ensembles. These overtones are felt often rather than heard and create a sense of harmonious feelings in the listener. For those that are sensitive to music, they create excitement in their use in a vocal ensemble. They express dynamics(louds and softs)which add to the dimension of a song emotionally. For one who is more sophisticated in music, unison singing all the way through a song can be boring. Harmonies are a way to express musical ideas that may enhance the text of a song.
To avoid harmonies in sea chanteys seems to me to be academically rigid. It seems to me that on shipboard, men engaged in hard labor would find relief in the music by instituting harmonies.
Shipboard recordings while men are working would be hard to do. When ashore,actual sailors would have to be counted on to provide information as to what happens when they are using chanteys for work.