[the phantom]

Terminal velocity for a human falling with his legs and arms stretched out is around 120 mph. That's pretty much as slow as you can fall.

Balled up- 200 mph

Maximum recorded free fall speed without any special equipment- 321 mph

That is really fast.

There is no doubt that such a fall would kill a person on land. Falling out of Celeborn and Galadriel's flet would likely kill someone.

Water is a different story.

Yes, it's true that water is more like a solid at high speeds (think skipping stones- or airplanes). I've gone water skiing before and yeah- when you try to flip and wipe out at 50 miles an hour you're going to skip a couple times and probably get a couple bruises, but it's not that bad. It's definitely possible to survive a 100 mph fall into water.

There was a jet pilot who bailed out at 10,000 plus feet and survived the dive into the ocean.

So no, the fall into the water did not have to kill or maim Gandalf and Durin's Bane.

But I bet they got a wicked welt, especially if they belly-flopped. Ouch.