Robert Horton was an American actor, born in LA, and may have never even heard the folksong or its pronunciation until the Hollywood writers produced that bastardized version for him to sing. Frankly, most Americans today are probably equally clueless about the pronunciation.

Incidentally, I wasn't speaking of the later sea shanty but of the original early 19th century folksong. I believe it originated with Missouri River traders and was about a native chief and his daughter. I guess you could call it a river song.

British singers can of course sing it in front of British audiences however they like and however the British like to hear it. I was just giving y'all a heads up about the way it sounds to Americans who are familiar with the folk song.