“When you’re talking about the impact of the water is how hard the water hits back. Really you have to be physically ready and if you’re not it will tear you apart.”
Don’t want to put a number on the height when it really starts to hurt impacting water but when you’re talking about freefalling from 90ft reaching speeds of 55mph, improper technique can not only lead to pain but serious injury. 
Cliff Divers in the Red Bull Cliff Diving World Series must enter perfectly vertical to withstand the force…or face severe injury and possibly the worst belly flop known to man:
