Marje: boats don't only transmit shanties! Look how many songs and tunes crossed the Atlantic. I think the sea route is most likely...fishing boats moved around with the seasons, so many people spent long periods of times based in Padstow when they actually came from far away. Conversely, many Cornish people moved in the 19th century in large groups to settle in other mining areas. Padstow, Liverpool Swinton is perfectly likely.
A singnificant point may well be the fact that many boats went to Padstow from far and wide for the May Day revels in the 19th century. So their crew would soon learn the May Song(it is in fact impossible to get out of your head after a day of incessant drumming in Padstow!)