WRONG.
If is sitting on a bottom that is not rocky, then that part of the sub is NOT subject to sea pressure, and it will be pressed down by sea pressure.
It is NOT being acted on by sea pressure from all sides, and this is very well known in UT.
Ever try to lift something off the sea floor at depth?
> If is sitting on a bottom that is not rocky, then that part of the sub is NOT subject to sea pressure
Correct. If it is sitting in, say, mud... then it's being acted on also by the "mud" pressure. Constant pressure all-round. While a circular cross-section sub may survive uniform pressure, it woudl only take relatively slight asymmetrical pressure to cause the hull to collapse.
> Ever try to lift something off the sea floor at depth?
Nope. But I can imagine that deep-sea mud can be as sticky as any other mud, and serve as glue. But the pressure difference issue is nonexistant.
the WATER pressure is equal all around even if it is on the bottom.
No, sorry - you're the one who's wrong. Picture a sub an inch from the ocean floor - equal water pressure all around it. It moves closer - still equal pressure. A millimeter away - still equal pressure. If what you say is true, then the INSTANT the sub touches the floor the pressure from the bottom vanishes, and the sub would be squashed like a bug.
With a 600 meter column of water pressing into the ocean floor, why doesn't the floor give way and get pressed down? Because the floor is pushing back with equal pressure. This is Newton's Third Law of Motion: for every action (the water pressing down) there is an equal and opposite reaction (the sea floor pushing up.)
You are correct that if the floor is muddy there, the sub is most likely lost. But that's because the mud is a fluid, and the sub would sink into it. Which is why it's so difficult to lift something off a lake or seabed at depth. It's the suction action of the muck, not the weight of the object. If you still don't believe me, here's a little experiment for you. Next time you're out diving, take a mechanical scale with you. Go down to where the floor is at 100' or so and place the scale on the floor. Take an extra 10 lb weight down with you and place it on the scale. It'll still weigh 10 lbs. I will bet anything on that that you would care to.