[TangoLimaSierra]

if you were to go down their and unbolt it It would never open.

Correct. And if you could open it the inrush of water would crush everyone.

Now, if it floated to the surface and no one found them, that would be torture as their O2 ran out.

[Equine1952]

Submarine outside hatches use sea pressure to help hold the hatch shut. The inner pressure has to equal outside pressure to open it. In scuba diving, Inside pressure has to equal pressure on the outside so you can breathe. You take a breath at 50 feet from a dive tank and go to the surface you have to exhale all the way up slowly. I was taught to follow the small bubbles. Bolting the hatch shut even surfaced pretty much screws the riders.