Couple of quotes about shanties -

Many a Chantyman was prized in spite of his poor voice becoause of his improvisations. Poor doggerel they were mostly and often very lewd and filthy, but they gave the knowing and appreciative shipmates, who roared the refrain, much opportunity for laughter
Frank T Bullen "Songs of Sea Labour"

....it was jovial, forthright almost wholesome obscenity..
Joanna C Colcord "Songs of American Sailormen"

Good luck with your quest :)