It depends, upon whether the van is moving ahead with a constant speed or accelerating.
If the van is moving with a constant speed, it is an inertial reference frame, and according to the principle of relativity, the laws of physics remain the same in all inertial reference frames, and hence the experience of the diver will be no different from a non-moving pool on the static ground, and he'll normally dive into the water.
However, if the van is constantly accelerating, then it is a non-inertial reference frame, and if the rate of acceleration were fast enough, the scenario shown in the cartoon would be possible.
All this is assuming that wind-drag and other factors are minimal and can be ignored. Otherwise, if there is a strong backward-direction wind blowing, that could also cause the scenario in the cartoon, even if the van were at a constant speed, or only accelerating mildly 😄
KISS: wrong, unless the speed of the vehicle is changing (a lot)
