The problem is that water is dense and a force is imparted on your body, quickly slowly you down as you enter the water. The faster you are moving when you hit the surface, the greater the imparted force. Falling from a higher height results in a greater velocity upon impact. If the force is large enough, and it will be from high heights, bones can be broken and organs damaged such that death is possible, and at higher heights likely. Suicide jumpers jumping from the Golden Gate Bridge in which the height is 220 feet have a 98% result of death on impact.
So how high can you jump into water and survive? It all depends on the orientation of your body when you enter the water. A belly flop into the water from 10-20 feet can be enough force to cause internal bleeding. Thus the importance of proper diving techniques, even from short heights.