doc tom, those seamen WERE the world wide web of their day, surfing from ship to ship and carrying their own take on the shanties of the era.
The variations of verses used compounded with the choruses attached, plus the particular 'tune' applied, makes any definative study almost endless in its conclusions.
Rules to singing shanties may have evolved, but they would vary from ship to ship and 'leader' to 'leader'.
If you were to state a concrete fact, given a few moments thought I could come up with a couple of examples to deny that fact.
Damned interesting, aint it :-)