Pop that sucker on the nose (I hear it's an effective method), or make his gills inoperable somehow. I've heard that sharks sink and die when they are not continuously swimming, so perhaps work with that. I hope this question didn't come from personal experience, or that you're looking to pick a fight with one. D:

It would depend on the size of the shark, assuming its a big bugger I would try and hit it in its eyes! Poke the eyes out and hope you get out of there fast enough before he comes back for a 2nd run! I also heard somewhere that lying very still can put them off so I might try that too.

How to Survive a Shark Attack: Remain calm. While you want to get out of water quickly if a shark attacks, you cannot outrun a shark in the water, and simply trying to sprint to safety may not be your best option. It's important to keep your wits about you so you can continuously appraise the situation and figure out how to get to safety. Sharks are highly instinctual, like dogs in a way: They have "fear" sonar and if you feel fear - even if not displaying it overtly - they sense it and that fear will stimulate their instinct to attack. It may take an instant black-belt, but your spiritual stance is very crucial. Keep your eye on the shark at all times. Sharks may retreat temporarily and then try to sneak up on you. Don't let this happen. To be able to defend against the shark, you must know where it is, so make every effort to watch the animal, even as you're trying to escape. Get into a defensible position. If you can't get out of the water right away, try to reduce the shark's possible angles of attack. If you're diving, back up against a reef, piling, or rock outcropping--any solid obstruction--so that you only have to defend attacks in front of you. If you're diving near the shore, you may need to descend to find cover. In open water, get back-to-back with another swimmer or diver so that you can see, and defend against, an attack from any direction. Gradually surface together to get back to the boat. Fight, Playing dead won't deter an aggressive shark. Your best bet if attacked is to make the shark see you as a strong, credible threat. Usually, a hard blow to the shark's gills, eyes, or--to a lesser extent--the tip of its nose will cause the shark to retreat. These are really the only vulnerable areas on a shark. If a shark continues to attack, or if it has you in its mouth, hit these areas repeatedly with hard jabs, and claw at the eyes and gills. If you have a speargun or pole, use it! Aim for the head, specifically the eyes or the gills. If you don't have a weapon, improvise. Use any inanimate object, such as a camera or a rock, or simply use your fists, elbows, knees, and feet to ward off the shark. If you have nothing around you, use your own body. Aim for the shark's eyes and gills. Use your fingers, elbows, knees, etc to poke at the eyes. Screaming under water will also work. The high pitched voice of a human will greatly affect the shark and cause it to move away. Get out of the water. While there are a number of things you can do to ward off a shark attack, you're not truly safe until you're out of the water. Your goal should always be to get back to shore or back on the boat. If a boat is nearby, call out calmly, but loudly, for them to come to you. Stay as still as possible while waiting--as long as the shark is not actively attacking you--and get into the boat as quickly as possible once the boat reaches you. If you are near shore, swim quickly, but smoothly. Thrashing will attract the shark's attention. Erratic movements may also give the appearance that you are wounded, and a wounded animal is more attractive prey to a shark. Use the smooth reverse breast stroke. This will reduce splashing. Get medical attention. If you've been bitten, get treatment as soon as possible. Massive blood loss will occur, depending on where you've been bitten, so immediately take appropriate precautions, (including, if necessary, the use of a tourniquet), to stop the bleeding. Even if your wounds appear minor, it's essential to get yourself checked out. http://www.wikihow.com/Survive-a-Shark-Attack

stay calm and pop them in the nose

If you are a shark expert, you can put the shark into the state of tonic...