For impact the usual unit to express the severity is the G-load. For instance, when cars are crash tested, the impact that the crash test dummies suffer is measured with accelerometers inside the dummies. Airbags can reduce the peak G-load to less than 10 times the load of normal gravity, allowing the passenger to walk away with only bruises.
In another answer to this question SE contributor D. Betchkal already pointed out an answer to a stackexchange question about force of an impact upon water
Stackexchange contributor Floris writes:
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.
Falling from large height in water the best case scenario is that you enter the water perpendicular, feet first. That way you penetrate deeper, hence the deceleration is spread out over more time, hence the force of deceleration is less.
Second best scenario is what happened to you; your spine was
perpendicular parallel to the surface of the water. (Had you hit the water at some angle severe spinal injury was very likely.)
The peak deceleration may have been very, very high, but lasting only a very, very brief instant. A quick calculation may give a reasonable estimate for your average deceleration, but there may well have been a far higher peak deceleration.
It seems to me the best indicator of the peak deceleration you experienced is the fact that you recovered from your injury without any medical treatment. A G-load over 100 G (no matter how short) is considered lethal. 50 G: possibly survivable, but with severe injury.