As soon as you've jumped into the corridor maze, face left. You should see a copper-colored switch. Pull it, flip around, and run/jump straight down the corridor. Hang a left at the end and leap through the temporarily opened doorway in the middle of the far wall (it will stay open after you make it in). Draw your harpoon gun and dip into the water. After the marine life has been filleted, swim into the main area and snag your swag. You'll probably use up a lot of health to get this prize, regardless of whether you try to kill all the fish or decide to evade them. Decide for yourself if getting the secret is worth the price.