Anyone who has spent any time at all engaged in strenuous repetitive manual labor knows that it is easier if you get into a steady rhythm. This is the case whether you are alone shoveling sand (or snow) or working with a team. And there is no better way to keep in that rhythm than with music in a slow rhytmic beat.
Therefore it is not a question of who originated the shanties. Anyone who has to work, in the old days before machinery, would have developed a singing work chant. This stands to reason on land or on the water.