According to an article in the Toronto Star, the final version of "The Wreck of the Edmund Fitzgerald" was indeed the first take of the song, and it was the first time Lightfoot and his band had ever played it together, and the album's recording engineer, Kenny Frieson, fortunately decided to record the performance down on tape because the band was playing the song through for the first time. The band rehearsed other attempts at recording it for two weeks afterward, but they were unable to capture the magic of that initial, unrehearsed performance, so the first take was used for the recording. In an interview with the Detroit Free Press, bassist Rick Haynes recalled Lightfoot initially only thought "The Wreck of the Edmund Fitzgerald" to be a short, unfinished song, but it was recorded when they had studio time left for the Summertime Dream album.^[18][19]^

Wikipedia

Learned that when I watched the CBC documentary about him. Still blows my mind.