system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people die from jumping off a bridge into water?
gpt
The answer is a combination of deceleration and water density (NOT surface tension as other answers say), with the latter causing the former.

When you hit the water at speed, you go from passing through a low-density substance (air) to a very high-density substance. Because it's dense, the molecules in it can't get out of the way when your body hits it.  This isn't a problem when you dip your hand in some water - the molecules are easily shoved aside - but it is a problem when you take that same hand and slap the water, resulting in it stinging because most of the water doesn't have time to move out of the way. So you feel the force of your slap just as if you swat a table. So the water feels much more like a solid surface, and the more of your body that contacts it at once, the more this is amplified, which is why bellyflops hurt so much more than clean dives. 

Now the deceleration part. You're going very very quickly from a high speed to a low speed, resulting in massive deceleration of different parts of your body. Besides bones possibly breaking because arms and legs get torn straight back or ribs get crushed in depending on how you land, your brain gets knocked around and the air gets knocked out of your lungs. These usually make you go unconscious or at least become massively disoriented, so you can't get to air and thus drown.

There can be other contributing factors like cold water causing hypothermia, or currents causing you to not be able to swim back to the surface, but these are the two "instant" ones.