I can't tell you the year, but sometime in the late '60's I heard a junior high-age girl with a ukelele singing Longfellow's 'Wreck of the Hesperus' to that tune. This was at a church talent show. I suspect the tune was used for that sad poem long before it became famous from Gilligan's Island. Otherwise the setting would have seemed incongruous.

The tune didn't become a folk song, it was a folk song.