Many of you seem to be implying that a shark skin is not tough.
Sharks have VERY tough skin. Unless you were stabbing things through with a knife on a regular basis, even adrenaline wont help you if you have a knife. I'm assuming you're all thinking of stabbing it through the brain, but that really wont work out too well.
Ideally, you need to go for the eyes and snout. This will deliver a sharp pain that will force the shark to back off.
I don't think sharks "bite on" like mammals do though. Their teeth grow sharp in every direction, and works more like a saw. They just open their mouth, and let their speed deal the damage as the teeth literally cut your limbs.
If they do bite on, I suppose thats the luckier scenario. Because then you can hit it. Otherwise it'll just swim by, take a chunk, and repeat.