I may have been wrong to say that Seeger initially thought the melody traditional (though it's a story told often enough); he himself later said just that he had used the tune without permission.

I'd go with the information at http://www.peteseeger.net/Icomeandstand.htm:

Original Turkish poem by Natzim Hikmet
English translation by Jeanette Turner
Music by James Waters ("The Great Silkie")
Adaptation by Pete Seeger (1962)
Text (c) 1966 by Stormking Music Inc.
Music (c) 1966 by Folk Legacy Records

Folk Legacy is Sandy and Caroline Paton, of course. With luck Sandy will spot this thread; but you might like to pm him about it. I'm pretty sure he's commented on this in the past, but not at the Mudcat.