Expansion of lungs makes us float because our whole body expands significantly, relative to our small volume.
In the examples mentioned above, the organs creating vacuum are said to be “somewhere inside” the body. Vacuum or not, Godzilla needs to visibly swell to increase its volume and buoyancy, which we don’t observe.
The air in submarines is used for pushing the water out of tanks, so the principle is ejecting matter. If Godzilla were to use that approach, as I said before, it needs to eject something.