I strongly suspect that verse was imported from another song--it occurs more fittingly, for instance, in "Ward the Pirate".

The historical figure, Andrew Barton, became High Admiral of Scotland and plundered under letters of marque against Scotland's enemies (notably, England). I doubt the real Barton would have disrespected the Scottish crown by proclaiming himself "king of the sea". So my inclination would be not to use that verse.