Water is incompressible. So the fall ends up being pretty close to 300 ft. to concrete vs. 30 ft. to concrete. Entering the water toes-first would just mean you have broken legs as well as being dead.
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!)
They did some stuff with cliff-style diving too (not quite that high, but it was good enough). That's one of many episodes that convinces me that they play at being more ignorant than they really are in the show, for dramatic purposes; it just isn't plausible that they do enough research to find a guy specialized in high diving, but somehow didn't find out what he could do before he came. I enjoy the show and I enjoy the results, but wish they would be a bit more honest at times, or be a bit more willing to say "Before we start, here's what the 'book' answer to the question is", because it's often obvious they have it. Also, editing makes it hard to tell, but it's pretty clear they very often come prepared with plans B and C to the offsites, again, making it pretty clear that they aren't as surprised that A failed as they play at.
It's the density and incompressibility. When you slap water, it hurts because there is more incompressible water in the way of the water you slap, not because the film on the surface of the water is holding together. Falling into water from some great height is pretty similar to slapping it. A smoother entry dissipates the energy of the fall over a longer period of time, reducing the experienced forces (hence diving).
