abfarid

Yes, but birds are very light in general. Most of their volume is feathers and they have a low bone density to boot. As the result, they have a very hard time diving, and have to either dive at high speed or paddle really hard to stay underwater.

And regarding boats, it depends. Do you mean completely empty passenger boats? Then yes, their density is very low by design, because they are mostly empty on the inside. When fully loaded, a commercial cargo vessel, is 80-90% under water.

First, let's address the expansion of lungs, because you say "little air", but in terms of volume, lungs are very big. On average, the volume of a human body is about 65 liters. When person fully exhales, the lung capacity is at about 1-1.5L; when expanded, it's about 5-6L. Interpreted charitably, that's roughly 8% percent of the entire human body volume. So realistically, expansion of the body by 8% is the difference between slowly sinking, and floating with the top of your skull (or roughly 1% of your body volume) peaking out of water.

Now, Godzilla, on the other hand, has like 80% of his body above water. Can you imagine, the amount of expansion that needs to happen for that much buoyancy? That's pufferfish territory.

So no, a "tiny percentage" increase in body volume driven by empty chamber "inside" his body would not be enough.

At this point, you're just trying to ridicule me over my choice of words and not actually trying to interpret them in the context that you yourself set:

they have a sack of muscles somewhere inside their body

Why mention "inside their body" if you didn't mean "deep" inside? All organs are "inside" the body. Therefore, I interpreted your words meaning truly "internal" organs, that that don't manifest themselves on visual inspection, like heart or bladder. Lungs, while technically inside, are peripheral and visibly expand - a critical distinction in this context.

So you specify "inside" and then mock my adherence to that framing, instead of addressing the core biomechanical issues being discussed.

Yes, that would work. But imagine the swelling, to give Godzilla that much buoyancy.

I made that assumption because lungs aren't really inside, they are pretty close to the surface, so they are easy to expand. If they were inside, they would have to push other organs away.
And regarding increasing the overall volume of the body, I addressed that in another comment. Basically, Godzilla would have to visibly swell by a lot, to have that much buoyancy.
It could be that the swelling is only in the underwater part, but then Godzilla would tip over with any slight movement, because the center of mass would be way above water.

I think I had this book as a kid.

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.

I just don't know why I'm getting booed, I'm right.

Exactly my point, the volume doesn't change in the example provided. Weight and volume stayed the same. We either need to expand Godzilla or it needs to eject some mass.

To get unnecessarily scientific here, that wouldn't change the overall density of the body, no? Even if there's now a cavity with vacuum, the matter that was occupying that space just moved somewhere else within the volumes of the body and the overall density remained the same.
Now, if it pushed some matter out, air or water, and created a vacuum cavity, that might work. But I'm not an engineer, so correct me if I'm wrong.

– You can't scare me with this Nazgul crap. I know my rights. I want my 2nd breakfast.

Looks like something you'd encounter in Morrowind.
...and you'd miss all the hits and get owned.