In chapter 14 of "The Sea-Wolf" we learn that Larsen has gone overboard and climbs back on board with a gash in his head. Johansen is nowhere to be seen or found. We are never really sure what happened to him. Larsen takes Hump down to were the crew is "sleeping" trying to discover who is really asleep. The lamp is put out and Larsen is attacked by several of the men. He fights his way out of the attack and climbs back up on deck. We can infere that Johansen was part of the plot to kill Larsen and that he hit Larsen over the head and as Larsen went over he took Johansen with him. However, this would only be a guess because the reader doesn't learn what happens to Johansen, he is gone.