It's possible to survive a free fall in water from extreme heights, but it's an extremely rare event, so the conditions for survival are not known. Snyder (1965) mentions two cases of survival following free fall in water after parachute failure:
- pilot falling from 15,000 feet in the Pacific Ocean
- sky diver falling falling 2,550 feet into a pond
Drag (thus velocity) could not be determined (by Snyder) in either case though.
After a bit more searching, I think the first case refers to that of Cliff Judkins who suffered a
streamer in 1963.
Given that falling a mere 59 meters (~193 feet) into water has a mortality rate of 85%, it's probably not a good idea to try this yourself though. As it was correctly pointed out in the comments below, this statistic is based suicide attempts...
... and it turns out that if you do find yourself free falling over water, Snyder found that body posture upon entering the water is a key factor in survival. His study included survivors only; although the majority of the cases were suicide attempts as well, a few accidental falls were included. He found
- No correlation between velocity and degree of trauma, but keep in mind all his data was for falls over 52 ft/sec = 35.5 mph, corresponding falling from a height of about 55 feet.
- Best way to increase survivability in water: land feet fist. Critical velocity in this position is about 100 ft/sec (110 km/h, 68 mph). Unlike falling on a hard surface, no fractures of feet or ankles were recorded in this position; bilateral midshaft fractures (i.e. breaking both legs) were common though, although the most common injuries were compression fractures of vertebrae for this posture. Organ damage occurred in only 14% or cases of entering feet first.
- In lateral or transverse (prone or spine) postures, 100% of the cases suffered internal organ damage, with renal hematoma being the most common one.
These findings are corroborated by a 1986 review paper that mentions that of all recorded cases (till then) of jumping from the Golden Gate bridge (250 feet high), all cases of survival landed feet first. The mortality rate was 100% for striking the water with the body horizontal.