As the Allmusic Guide puts it so well, "It was never supposed to be like this: '(Sittin' on) The Dock of the Bay' was supposed to mark a beginning of a new phase in Otis Redding's career, not an ending." Posthumously released in January of 1968 after Otis' death in a plane crash in December of 1967, this radio-friendly change of pace for Redding was immediately added to R&B radio station playlists.
Redding co-wrote the song with guitarist/producer/songwriter Steve Cropper. It's easy to fall under the spell of Otis' vocal so don't miss Cropper's subtle guitar work.
Dock of the Bay was the first posthumous number one single in U.S. chart history. The tune shot to number one on the Billboard R&B charts and number four on the magazine's pop charts. Moreover, the song has lasted. BMI named it the 6th most performed song of the 20th century. I saw evidence of this just recently, when a great cover of the tune got a big round of applause from a seemingly disinterested audience at a bar happy hour. It's part of the culture now, and I can't see it going anywhere. Otis and Steve, thanks for this classic.
[This post also contains information from Wikipedia's page on the song.]