Impact on water is a very complex topic.
Your simple calculation just figures out the velocity of a free-falling body after a 50 m drop. That just tells you the initial relative velocity of body and water surface. It doesn't tell you much about the force at impact, or whether the person survived.
There are two things that might kill on impact: high local pressure on the surface of the body - this can cause lacerations, bleeding, fractures; and a deceleration of the body that causes internal organs (the brain or the kidneys) to rip loose, resulting in severe internal bleeding and death.
The key to survival, then, is to minimize the local pressure and the deceleration. This is why you see stunt men dive in the most streamlined manner possible - they try to minimize the area with which they enter the water, which limits the force on the body and thus maximizes the time to decelerate. It also helps to fall into "less dense" water - for example, the frothy water right behind a cruise ship will have a lot of bubbles in it, and this will increase the distance you move before slowing down.
There is an additional complication which relates to the shape of the contact area - you may be familiar with the "belly flop", where you fall flat on the water and it hurts a lot. This is not just because you slow down quickly - it is because there is a brief moment when the contact point between your body and the water moves faster than the speed of sound in water, and this results in an "attached shock wave" which can cause the pressure of the water to briefly become very high (see for example http://www.dtic.mil/dtic/tr/fulltext/u2/a259783.pdf - a bit of a long paper...).
It seems that the world record for a stunt man surviving a fall into water stands at just over 54 m (Olivier Favre, https://www.youtube.com/watch?v=mLd529gWKJ4). And that is "still" water. As I mentioned above, in frothy water it might be possible to survive a higher drop.
At the same time, people frequently kill themselves by jumping off the Golden Gate bridge - about 70 m height.
Found an interesting link discussing this - the last answer sounds quite credible in its analysis, making some attempt at quantifying the forces during impact. I don't agree with the entire analysis (in particular the "slamming into a stationary body at half the speed" is too much of an approximation for my taste) but it makes some good points: https://wat.lewiscollard.com/archive/www.newton.dep.anl.gov/askasci/gen01/gen01790.htm