Sherlock Holmes fancied himself a pirate when he was young, and might have carried a bit of that whimsy into adulthood. And why not? Venturing out on your own, outside the traditional maritime laws . . . if Sherlock Holmes was not the pirate captain of Victorian detectives, I don't know who was.
What does Holmes say about pirates in the ACD Canon?
"It is quite clear that the Colonel is a cool and desperate men, who was absolutely determined that nothing should stand in the way of his little game, like those out-and-out pirates who will leave no survivor from a captured ship."
From his words in "Engineer's Thumb," one would presume Sherlock Holmes had mentally categorized levels of piracy, and "out-and-out pirates" were the most ruthless of all. And if Sherlock was going to the trouble to parse out his pirates, then chances are there were some pirates he felt a certain empathy for. And rightly so, growing up in a nation where Sir Francis Drake was a very famous pirate.
Tales of Tortuga, Morgan, Blackbeard, Kidd, and the like may well have been the very first criminals to catch the attentions of a young Sherlock Holmes. His first studies of crime could likely have been an attempt to read Alexandre Exquemelin's History of Buccaneers of America from the family library. Before a child named Sherlock Holmes decided to bring modern science and intellect to the study of crime, setting his sites on improving methods of piracy and running them through playground testing against an elder brother who, already, was the British government in Sherlock's eyes . . . or at least the British Navy.
There is something about Sherlock Holmes's style of detective work and dispensing personal justice where needed that make a pirate's life something his heart might have held a certain affinity for throughout his life. Retiring so close to the shore, spending time on the beach, and writing his one retirement memoir about an invader from the sea are facts that lean more than a little bit toward such a theory. But knowing that there were "out-and-out pirates" out there, even on land, who needed to be dealt with gave him a more noble and realistic purpose than hunting treasure-ships on the ocean's waves.
These are waters, I think, that call for a little more exploration.