I think it's 'down the marsh'; you lock out of the floating harbour into the tidal River Avon which is quite shallow and muddy and, as Nigel said, somewhat sinuous. (There's a picture of it with the video.) But why 'marsh' I don't know. Maybe a local name for a stretch of the river above the gorge.

The juxtaposition of 'bollard', which is on shore, and 'mast' which is aboard ship, is odd. Does he want to go or not?

Just south of the harbour is the church of St. Mary's Redcliffe, with its spire and bells, also mentioned in the song. One of the nicest-sounding rings of 12 bells I know.

And it's Klondike shore. So he's joining the gold rush. But elsewhere he says he's going to sail round the world.

"Goodbye to you in ??? wells" No idea. Wells is a nearby town in Somerset; maybe there's somewhere called 'somthing Wells' - or maybe it's a pub.