Quoted from Joost

I know it probably gets annoying by now that I keep telling everyone they're wrong, but this is a myth as well. It is reported that the Beach Boys already performed the three chord folk version of Sloop John B in 1961. Al did come with the suggestion of turning it into a Beach Boys song in 1965 (and 24 hours later Brian already had the backing track recorded), but Brian was already familiar with the song.