water in tunnel at the end

It's possible, but you might be getting forced under water at certain point, or at the end where you come out. When you have no control, it's a lot harder to stay afloat. That said, why would Heston have even bothered when he could have guessed the chances of survival are less than 1%?

Not likely. It seems that every year people drown in flood control channels during severe downpours, and those channels have no ceiling to cut off one's air supply. If people are in the water for even ten minutes the cold water can cause hypothermia and loss of strength to keep head above water.

People asking questions like this apparently don't understand the dangers of rushing flood water.

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Whose idea was it for the word "Lisp" to have an "S" in it?