It seems to me the surface tension is the deal-breaker here. When you land feet first, your feet break the surface of the water, which allows for the water to move around the incoming object. You want to minimize distribution of force to only the bottom of the feet.
Of course the force exerted on your feet would more than likely break your ankles and possibly your knees/legs, and severely impinge your hip joint. In addition, your head will likely receive an extreme "punch" as it comes into contact with the water. This alone might be enough to knock someone unconscious or kill them, but it depends on their neck, chest, head, body position, etc.
So if the mythbusters test were done again, it should definitely not be a really fat pig landing on its side; it should be a skinny dummy landing perfectly straight.
I am not a scientist, but I do have experience with cliff diving. (The real trick is when the water level is low, you have to expand your body as soon as you hit the water so you don't shoot down and hit the rocks at the bottom!)
