I doubt that the 107 year old Cape Horner was making songs up on the spot out to sea, perhaps in a shipyard but not out to sea. As for singing partners- well, entertainment is a another matter. If you have actually done capstan or halyard work you would know where reality is located.
Try actually working at these jobs and making songs up on the spot-it can't be done effectively. Just because you say it's so don't make it so.   
Rap music is pure entertainment. It has nothing to do with work and so lends itself to spontaneity.