Charley,
I'm with you there. I'm as happy belting out a shanty as I am warbling a Child ballad.
And as a singer I see no need to stick to the printed page or the songs I pick up from other singers.
I assume the best and most treasured shantymen were those who improvised their verses and had a good dig at those not heaving or pulling.
I know I'd pull twice as hard if I was listening to the mate being slagged off or the shantyman's latest conquests ashore.