Waly is Scottish for Wail, and the long history of this song would make most scholars wail! But Jürgen Kloss, a folklorist from Bonn, Germany has really done it here: http://www.justanothertune.com/html/wateriswide.html
It's a loooooong read, and mostly about the 7 zillion different versions of the words, but with interesting versions of the tune, and how it came to be attached to the words of the song as we know it now.
He also has a long history of The Parting Glass, which I do mean to set a nice arrangement of soon - http://www.justanothertune.com/html/partingglass.html
Historians note - this man has en eye for detail!