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The Mythbusters test how deep bullets fired into the water are still lethal. They ran tests with a few other rifles but the test shown here was the .50cal BMG. The conclusion was that you would be safe if you were deeper than 14".
Liquids cannot compress (unlike air or other gasses), they can only displace. As a result, they become more like a solid when struck at a very high velocity. This is why the lower velocity rounds performed better. The higher velocity rounds simply tear themselves apart.
what causes the bullet to shatter like that? maybe armor should be made of water :p