I remember it as "down" from the sixties, for what that's worth. I also remember a singer in Blackpool Club, mid seventies, introducing it as a Yorkshire song. I was sitting between two lads from Grimsby, who put him right very quickly!

I would offer one piece of evidence, not directly relevant perhaps but indicative. On British railways, the down platform was always away from London. So heading North (as from Yarmouth to Scarborough) would be going "down". The point, perhaps, is that geographical "south" is not always "down", despite that being the way they print the maps. That's just a misapprehension of the chairborne.

I think the true reason lies in the sailing conditions, but I've no direct evidence.