This is a sea chantey that got distilled, and transformed into a love ballad in the Appalachian Mountains. The origin is Scottish, but the lyrical style is obviously from the Southern United States. Many settlers to the New World brought their music with them, only to have it subtly changed over time.
Another example of this phenomenon is Jean Ritchie's song 'Nottamun Town,' which only survived by being brought to North America. When, as a Fulbright Scholar she visited Nottingham, England to research the roots of the song, it had completely disappeared in its original form.
Appalachian Traditional Music, A Short History: http://www.mustrad.org.uk/articles/appalach.htm
Lyrics and chords available from Folk Den Project.