Simple physics holds that the larger the surface area of the body hitting the water, the larger the resistance force of the water pushing back will be. When your body falls flat on the water from a distance with a bit of speed, it creates a big impact that can feel a bit like falling on concrete.

How far do you have to fall for water to hurt you?

Stone states that jumping from 150 feet (46 metres) or higher on land, and 250 feet (76 metres) or more on water, is 95% to 98% fatal. 150 feet/46 metres, equates to roughly 10 to 15 stories in a building, depending on the height of one story.

Is hitting water like hitting concrete?

Pressures caused by breaking the surface make water act more solid on shorter timescales, which is why they say hitting water at high speeds is like hitting concrete; on those short times, it is actually like concrete!