If you have watched a whip being cracked you know how softness relates to speed.
A whip can only crack because it is flexible and soft, and in many cases tapered, this allows the force of the wave to be focused into a smaller and smaller area, resulting in an increase in frequency and speed of the wave form, the end result of course being that the movement of the whip itself exceeds the speed of sound.
With the water hammer phenomena the pulse of energy is propagated through the medium, rather than the medium moving with the pulse. With the whip the pulse is propagated by the medium moving with the wave. Both involve fluid dynamics, the whip acts in a fluid manner, so does the water. However while the whip moves with the wave, in the water hammer the motion of the wave does not involve the motion of the medium as a wave, the resulting difference being that the waterhammer pulse propagates at a much higher frequency than the whip is capable of entailing.
The power and velocity of the whip relates to how soft it is. If it is too soft it is hard to actuate, but if it is too stiff it will not be effective.
Many martial arts emphasis whip like movement involving a snap like recoil. This allows the move to have whip like impact. While this is effective, it is not the same as the water hammer effect where the pulse of energy is propagated internally in the medium while in the whip the medium itself moves with the propagation of energy.
With the whip the maximum force comes from the junction between the extension and the withdrawal phase, however the water hammer effect does not entail a withdrawal phase, it entails what looks to the eye to be a type of pause, in which the energy of momentum becomes concentrated and transmitted at a higher frequency than the initial movement. In a whip motion there is a visible withdrawal aspect, thus there is no pause for the energy to transmit, this has to do with the whip method entailing impact as it transmits force and the water hammer entailing established contact prior to transmission of force. Even if the whip is done in contact the effect will not be that of the water hammer effect because the dynamics involved are different.
Both the whip and the water hammer require a type of softness to work, however one employs stillness and the other does not. The water hammer uses stillness to change the surge of energy into a higher frequency, while the whip does not. Instead the frequency of the surge of the whip is increased by the taper of the whip which concentrates the motion into a tighter area, similar to the water hammer in principal but still worlds apart in manifestation.