Escaping From A Sunken Submarine: Is It Possible? – SlashGear

The biggest factor in escaping a stuck submarine is how deep you actually are in the ocean — because the likelihood of human survival drops significantly the further you go. While it may not be impossible for a person to leave a sub and make it to the surface alive from a depth of around 130 feet (40 meters), it’s about the average for safe recreational diving. Any deeper and ever-increasing water pressure could become too much for your body to handle.

Similarly, the farther down you are the longer you’ll have to hold your breath before getting to the surface of the water. Additionally, the water gets much colder as you go deeper, regardless of the above-water climate, so there’s also the potential for temperature or thermal shock, which could make moving and thinking clearly more difficult.

But if someone was trapped in a sub that was roughly 100 feet or so underwater, making an unassisted break for fresh air is possible. Assuming the vehicle has a hatch that can be opened from the inside, you’ll need to create a large enough gap between it and the hull to let water in. Then once the pressure inside the sub matches the pressure outside (i.e. the sub is filled with water), you should be able to push the hatch open and swimming or float to the surface.