After you have descended 32 feet, the water at one atmosphere of pressure will press against that hatch as 1 ton per square foot. How do you think you are going to push the hatch open if it is unbolted? Every 32 feet is another atmosphere of pressure.
You don't, because that's not what it's designed for. Submarines that are designed to allow exit at depth have airlocks or moonpools, which account for that pressure differential
Why would anyone get into something that's even just bolted once? From the outside? With no way to get from the inside??
After you have descended 32 feet, the water at one atmosphere of pressure will press against that hatch as 1 ton per square foot. How do you think you are going to push the hatch open if it is unbolted? Every 32 feet is another atmosphere of pressure.
You don't, because that's not what it's designed for. Submarines that are designed to allow exit at depth have airlocks or moonpools, which account for that pressure differential