Terminal velocity is only part of the answer.

What matters is how fast you impact the water. The speed at which that is fatal isn't really anything to do with terminal velocity at all. After all, if you die when you hit water at 200kph does it matter whether you're going 201 or 500?

What's more interesting in this question is how water reacts when you try to break its surface: you can dive from a great height and be fine, but go a few metres higher and the water can't 'get out of the way' quick enough so you end up smeared on the surface. Obviously the shape of the object falling has an effect - pointy is more likely to break the surface than flat, etc. I'm sure the Navy has done some studies on torpedoes dropping from planes or something that would give us a hint as to the physics involved.

Come on - inquiring minds want to know.

If I remember right, divers in competitions dive into a pool which has air bubbling up at the approximate point of impact to break the surface tension and to lower the density of the water at that point.

You would find a similar effect at the foot of the cliffs that the cliff divers use in that wave action disturbs the surface tension and causes a water/air emulsion which is less dense than still water.

I suspect that if the cliff divers tried the same stunt into a perfectly still pool, they would be coming out with more injuries; broken hands, fractured skulls, dislocated shoulders, etc.

So there are several variables affecting the potential fatality of a fall into water; velocity, surface tension of the water, density of the water, the attitude at which the body encounters the water and probably the ability of the body to absorb the shock of the encounter without causing fatal injuries. Possibly a fat person could survive a fall that would kill a thin person because of the shock absorbing qualities of adipose tissue?

I've always maintained that terminal velocity is survivable if you get it right. I would like a pound for every "water's like concrete" comment I've had in response. Anyway, two interesting links, the second one supports the idea that terminal veloicty is survivable.

http://www.wikihow.com/Survive-a-Long-Fall

and referenced from that:

http://amelia.db.erau.edu/reports/faa/am/AM65-12.pdf

so you didn't find the answer?

you should take an estimate then, I would think it involves surface tension of the water, combined with the most optimal angle of attack to survive, I think you should have been able to approximate the square inch implication and translate that into a rough guess to what speed would be survivable

give it another go