Bioshock Infinite

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Except it isn't. There would be less pressure on your ribcage then, say, your abdomen or neck.

...why? Pressure distributes evenly in liquids. Its not the same as being crushed under rocks. (Thats why fragile animals like jellyfish are able to live down there.) The only difference in pressure when you're standing upright and completely submerged in the water would be the height of your own body, which is small enough to be negligible.
Also, like I said, the only thing that would get compressed is the air in your lungs. And only if you held your breath after breathing in air that has a lower pressure than the water.
(That's why breathing pressurized air underwater works - the air has the same pressure as the water surrounding it, thus the pressure inside your lungs and the pressure coming from outside is the same and you can breathe normally. Using a really long snorkel on the other hand would be impossible, because the water pressure would prevent you from breathing in.)

What would kill you of course is the force of the wave crashing through the tunnel after something damages it. But thats a different problem