One thing that interests me: how does denpressure act underwater?
Certainly there is air in water, but it doesn't seem to permeate in quite the same way. It's easy to tell that it would take some force for air to be pushed down to the bottom of any chamber of water (just blow on a cup for an easy test). If it just seeps through, though, how would it interact with the natural buoyancy of water?
What we have to remember is the objects ability to completely trap atmosphere as well as simply absorbing it. This defines how it works in water.
It's always high and low pressures and if an object has low pressure trapped in it, it will be SQUEEZED down but if it has high pressure of TRAPPED atmosphere, it will be squeezed up.
There's two things going on in objects. One is initial absorption of atmosphere and the other is the trapped atmosphere within that only energy applied at depth could release it from, or heating and super cooling.
For example, something I remember doing in a swimming pool countless times, there are a lot of objects you can push underwater and find that they don't sink or rise. There are also objects that float, and if you push them under they force their way back to the top. That's easy to understand in terms of buoyancy.
But what about the objects that sink? How would denpressure act on them when underwater?
By slowly trying to squeeze. You see the water is in direct resistance with the atmosphere above it, because water is simply just super small atmospheric molecules, but top us they're dense enough to be liquid due to its own stack from the bottom up against the atmospheric stack from the top of the water to the top of the sky.
Basically the object sinking is being squeezed but you can't see that initially, just as you can't see a heated train wheel expanding by heat to fit a cold inner hub, but you know it's happening when you see it fit on easily and then cooled off to be as tight as a weld.
Whatever air molecules are there are all mixed in with the molecules of the water, so displacing one would displace the other. At any stage, the object's displacing a lot of molecules, but it's entering into the pool from the top so it ought to be pushed back up by the water.
There's a few things going on. One is the squeeze of the water that releases some atmospheric molecules, making it smaller as well as temperature change as the object is squeezed.
The two solutions I can see, that don't seem to work, are:
The water and air displacement forces acting inside a body of water are opposed. Air is still being displaced ground-up so it pushes down, water is being displaced top-down so it pushes up. My issue here is that surely the water and air would act on each other, in this case, evening out the direction of force? (And there might be issues in terms of heavier objects sinking when the water-force ought to be stronger as they displace more, but I could be wrong).
Hopefully I've gave you food for thought but if not, come back at me and I'll try again.
The second option would be that an object that sinks does so because it was in the air, and didn't lose the impact of that force, so it was pushed down to the bottom and the water couldn't immediately act to push it back up (and, once in the water, the water acts on the pores to reduce the displacement and so reduce the upwards force). Presumably though that wouldn't last forever, and eventually the downwards force would be worn away by the upwards, and it'd float, which doesn't seem to match what we see.
Imagine putting a block of wood on top of the water. A soft porous wood. At first you see it sitting proudly on top with the majority of it's area protruding from the water.
The water is pushing against the sides of that block and by doing so it's pushing it up, or resisting it, because the atmosphere is doing exactly the same thing to it. It's a game of push of war instead of tug of war.
The water is more dense than atmosphere and we see that by the block only submerged in a minor way.
We know the wood is porous and is full of atmosphere which means that the atmosphere is more pushing through the wood rather than pushing on it by a squeeze DOWN.
All it can do is keep playing push of war until bit by bit the water density is overcome a little, meaning more density of wood to push back against and so on until over time the atmosphere wins by squeezing the wood down into the water and now squeezing or pushing down on the water with the water pushing back to now work of squeezing the trapped molecules, leaving the wood super dense compared to what it was.
The first option seems more likely, but I can't quite see how it would work. There might be an interesting test in terms of water-behaviour though.
Let's see if I've said anything that can peak your interest or (hopefully not) whether I've complicated the issue.
I'm willing to keep trying to make better analogies and what not to keep you on track because I know you're really delving into it and trying to figure it out.