Well, the tricky thing about "The Secret Sharer" is that the reader never really knows for sure what Leggatt is. Joseph Conrad, the author, was an avid fan of Sigmund Freud. Although much of Freud's philosophical and psychological theories have since been debunked, at the time he was very popular. Freud believed that our dreams were our deepest consciousness, and if the reader considers "The Secret Sharer" to be a dream of the captain's, then Leggatt is a sort of second self. The problem with the captain of the ship, who, interestingly, is never named, is that he is relatively new to his post. The deeper question, then, is who the captain of your own personal ship is. There are a lot of confusing images and themes that keep popping up over the course of the story. Darkness, for one, definitely ranks fairly high. It is a broader theme for more specific images all tying back into Leggatt. The black scorpion, echoing Leggatt's black hair and perhaps signifying that he is dangerous, would certainly fit with an idea of Leggatt being real; Captain Archibold is searching for Leggatt on the grounds of a murder charge. The captain finds Leggatt in the darkness and consequently hides him in the dark parts of his cabin. When Leggatt, assisted by the captain, lowers himself into the water to swim away, it is also under the cover of darkness. It seems like a fairly simple metaphor, darkness for hidden and dangerous things, but then we come back to the idea of a dream.

If Leggatt is a fictitious creation of the captain's mind, whether dreaming or not, the darkness metaphor takes on a different meaning. Instead of danger, it is the haziness of an unknown future. As mentioned before, the captain is not introduced as an authoritative figure. The story can be seen as a kind of bildungsroman, or coming of age story. By the time the story ends, the captain has become much more assertive. He commands the direction of his ship through the rocks. The main theme of the story becomes exploration of the self. It is done expertly with a doppelgänger: Leggatt as the second self of the captain. Literature is often not like math— there is often not just one right answer—and the question of whether Leggatt is real likely cannot be answered by anyone but Joseph Conrad himself. However, it is quite easy to come up with fairly solid evidence toward the theory of Leggatt being some sort of dream—the inner consciousness of the captain that just needed some help to come out. Perhaps it was the captain, not Leggatt, who was the strong swimmer all along.

Strong arguments can be made for both sides.  No one else sees Leggatt, so the white figure in the water that is naked as a newly-formed human would be is "born" as the captain pulls him from the water.  Gazing into the dark waters, the captain may have psychologically realized that he has a dark side and, for a time communicated with it.  (At one point in the novella, the pronoun "it" is used to refer to Leggatt, in fact.)  Then, he has again hidden this darker side by resubmerging it in the depths from which it came.  On the other hand, Leggatt may, indeed, be the first mate of the Sephora, whom the captain saves and hides in his cabin because he sees something in him, and because he is solitary.  But, fear of the consequences of harboring a murderer may cause the captain to send Leggatt back to the sea.