A submarine sinks due to displacement septic. So do we now have underwater dunny pressure or can we just go back to the gravity idea.
I'm pleased you brought this up because it helps to show why dense objects sink or displace more than others.
A submarine sinks due to allowing atmospheric pressure to push water into it to give it more density. The more water you alloe into it, the more it sinks.
The thing is, it can only sink if it takes in water and the more water acting on the air inside of it, creates a crush against that air, as air compresses.
As soon as you stop water being allowed into the sub, it will simply stop sinking or rising. It will hold a depth.
The only way you can sink it, is to keep taking in water from the water pressure build up from above that is pushed in by the force of atmospheric pressure on top of the water that is constantly pushing down.
As you well know. A sub will be crushed if it goes too far down due to compression of air inside not able to withstand the outside pressure, whch will breach the hull and force out the air back to the top of the water to equalise with the atmospheric pressure above the water.
No matter what you can think of. Atmospheric pressure is they key to it all. No gravity involved.