If you shoot with normal bullets to diver, you have no chance to kill him. You can only if he is standing with head above the water. Bullets should go few meters through water (with less strengh) and then stop.
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Let the bullets (from normal guns) go a few meters through the water with reduced strenght.
For better understanding, here is video on youtube http://youtu.be/PXBwYrzVAHs
Actually that would be the realistic and expected behaviour of a round entering water. In fact the higher the bullet velocity then the less damage it does, as the water does in fact act like a solid object and will first disintegrate the bullet before slowing it to a complete stop within inches. So oddly you would have a better chance of killing something submerged in water in real life with a 9mm pistol than a 50 cal HMG. Even then the damage would be minimal in most cases.
It would be interesting though to see how a grenade or any explosive works in that situation as explosives use the generated water pressure to damage objects e.g. mines and depthcharges on submarines.
[EDITED TO ADD] Found this clip on You Tube by Mythbusters, which demonstrates how water makes a great bulletproof vest. http://www.youtube.com/watch?v=v1uaLWAZXfk