I've been researching and posting information about our Paul's real name and I am convinced that his real name was JOHN BLACK. It makes sense that the competitive Brian Wilson of The Beach Boys would want to make some comment on John Black's life and I think "Sloop John B" was it.
For right now, I can't say how biographical the song is. I am researching the probability that at some time in the 1965-1966 time frame The Beatles owned--or had access to--a yacht.
And I did find a very interesting quote from Geoffrey Ellis, who was the Chief Executive of NEM Enterprises, Beatles' manager's Brian Epstein's company. In Ellis' 2006 book, I Should Have Known Better: A Life In Pop Management--The Beatles, Brian Epstein and Elton John, he says:
"As well as dealing with contracts, tax matters and various company administrative affairs, there were concerns over aspects of the Beatles' lives which required attention. The secrecy over Paul's whereabouts when he went on holiday, awkwardly for the preparations for the Northern Songs stock market flotation, was typical of the discretion which always had to be exercised over the movements of each of the four Beatles, and over all aspects of their private lives." (my emphasis)And an interesting note is that--as I said in the 2015 post--The Beach Boys hired Derek Taylor who had been The Beatles' publicist, as their publicist. Derek Taylor directed an innocuous little video featuring The Beach Boys goofing around in a swimming pool with "Sloop John B" as the video's theme music. But the song's title--and possibly some of the song's lyrics-- was a sly reference to our Paul.
