On physical buoyancy

In fact, the submarine changes its own gravity by absorbing water and releasing water during its voyage from the Yangtze River to the East China Sea. Its purpose is not to surface.

In fact, if the water put into the cabin is counted as the mass of the ship itself, then the buoyancy of the ship in the East China Sea is greater than that in the Yangtze River, but the gravity of the ship in the East China Sea is also greater than that in the Yangtze River, so it is still suspended.

If you want to compare, the East China Sea is more buoyant.

It should be different from a boat. As far as the ship is concerned, the buoyancy on both sides is the same, because the draft on both sides is different.