Drasiana, the "you" in that verse is the "you" who bought him the drink. I'm pretty sure that he (the narrator) became a/the black swan, (Grim Reaper/Cygnus/Whatever), or thinks he did and is now telling this story in a pub somewhere. And the "you" from both of those verses is the "you" from the very last line... who then becomes the black swan.
Drasiana, the "you" in that verse is the "you" who bought him the drink. I'm pretty sure that he (the narrator) became a/the black swan, (Grim Reaper/Cygnus/Whatever), or thinks he did and is now telling this story in a pub somewhere. And the "you" from both of those verses is the "you" from the very last line... who then becomes the black swan.