a previous answer got it right, but if you still dont understand, think of it like this.
the frame of cars (hood, doors, bumpers, etc.) are built to crumple and deform in a crash. this lets the force of the crash be distributed over a larger area, and over a longer time. thus decreasing the magnitude of the force at any single moment.
also, note that if one enters the water as a diver would, in essence it's still going to displace the same amount of water eventually, however over a much longer period of time. which is why the height from which you fall, nor the speed when you land is most important.
the most important is how well you can position your body to maximize the amount of time it takes for your body to completely submerge.