Water has a lot of mass and surface tension. When you hit it at velocity, the g's from the impact can be fatal. People who cliff dive purposely land with a very narrow profile to minimize the force of the impact.

To feel the forces of the water do a belly flop by jumping from just above the water's surface and you will notice the water can have quite a high force on you and take time to move away

Workers on the Golden Gate Bridge always kept their toolbelts loose and quickly detachable so if the fell, they could throw it while falling to break the tension, thus saving them for the surface splat

Water "takes a while" to move out of the way of an object that impacts it. And gravity accelerates you quite strongly on your way down, so from higher bridges you'll have quite a bit more speed than from lower bridges.

So bottom line, the faster you go, the more water will feel like a thick syrup, very slow to get out of your way and make room for your body, compared to your speed. The impact with the surface feels more and more like an actual impact, like hitting a solid surface. Such impacts cause fractures and internal damage, you can die from them.

Just to give you an idea, we do this with AIR too, that's what an explosion is. You can see an explosion's spherical shock wave, that is a wall of air atoms that are moving so fast that they are not going to get "out of the way" of any object in their path. The shock wave is just air, but it impacts with enough force to destroy buildings and kill people.

This sounds like the kind of thing that's completely made up, but nobody can really disprove.

It is, but it's not un-disprovable. Mythbusters did this and found that it doesn't really help. The tools break the surface tension, but the surface tension isn't the only factor, and the water still hits you like a truck.

un-disprovable is a brilliant new word.

also bless the mythbusters and RIP Grant

Honestly sounds like a word Trump would use.

nah he'd say "undidstroopable" or something