What's not clear is how deep the water is up until the point they fall in. It's pretty obvious from the fact that they always get the same distance before falli

What's not clear is how deep the water is up until the point they fall in. It's pretty obvious from the fact that they always get the same distance before falling that "something" is different up until that point. Either the water up until that point disobeys the laws of physics, or there is a walkway just under the surface of the water that they're running on.