I was also wondering about the tune after being at a session last week where someone sang "My (or 'The') Lovely Rose Of Clare" which bares a striking resemblance to the tune used for "3 Score & 10" & where there were slight differences it was still in harmony though it had no chorus just a repeating refrain. I have no idea of the age of "My (or 'The') Lovely Rose Of Clare".
About the "Down", I believe the "Down" as I've always seen it in print (Roy Palmer has it as "From Yarmouth 'Down' to Scarborough", see below) & heard it, referes to sailing downwind, doesn't matter if it's up on a chart or north if it's downwind it's down as far as a sailor's concerned.

I believe we've done this before, from the Oxford Book of Sea Songs by Roy Palmer (p.274-275).

"'In Memoriam of the poor fishermen who lost their lives in the Dreadful Gale from Grimsby & Hull, Feb. 8&9, 1889' is the title of a broadside produced by a Grimsby fisherman, WIlliam Deld, to raise funds for the bereaved families. It lists 8 lost vessels, the last 2 from Hull: Eton, John Wintringham, Sea Searcher, Sir Fred, Brittish Workmann, Kitten, Harold, Adventure & Olive Branch. I addition the names of some of the lost sailors are given, & there is a poem in 8 stanzas. This past into oral tradition, & in so doing lost 6 verses & aquired a new one (the last, in which an error of date occurs), together with a chorus & a tune. The oral version was noted from a master mariner, Mr. J. Pearson of Filey, in 1957, and has subsequently, with some further small veriations, become well known in folk-song clubs."

Barry