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.

I can't, and I wasn't going to. My argument was never about what Godzilla can or can't do, it was about physics. Specifically, that you can't move stuff around internally, without changing volume significantly, to change buoyancy.

Deballasting bone cavities is definitely an option. But to achieve the levels of buoyancy displayed by Godzilla, they'd need to be truly massive. Or he's using paddling in tandem to help itself stay above water, akin to what dolphins do to hold most of their body above water.

Also, you can't squeeze bones, so Godzilla needs an organ that would force discharge that ballast. Like sacks of highly compressed air, which are used to push out the water completely. This is similar to what submarines do.

Instead of bones, we could also just use your approach with organs. Emptying sacks of water and filling them with air. But either way, we need to discharge ballast, as I was saying originally. It's a limitation of law of physics, and not a limitation of Godzilla's abilities.

Source: I have a bachelor's degree in Maritime Transportation and Navigation. Which is basically a BSc on "how to buoyancy right".

Don't know what to tell you, man. You sound very confident, but I literally have a bachelor’s degree in Maritime Transportation and Navigation, and have served on several cargo vessels, as well as a couple of passenger ferries. I might have exaggerated with 90%, I'll give you that, so take it down to 80%.

cruise ships are 10% underwater

As I said, those are usually mostly above water, to prioritize comfort. But even those are at least 30% underwater, with very low center of gravity. You can't have a ship 90% above water; it would keel over. Except some heavyweight barges that have big surface area, I suppose.

A fully loaded cargo ship can’t be more than 30%

In fact, that's about the least an empty cargo ship is underwater. Because when empty, cargo ships take ballast to prevent capsizing. Also the propeller is designed to be at least a few meters below water to be effective.

they tend to be stacked far higher than the ship’s sides

I think you're focused specifically on container vessels. Those still have way more massive holds than the containers you see on deck.

Ocean waves would easily swamp a ship that was 80-90% underwater

Depending on the season and projected weather conditions, ships are leaded to a different extents. They have load lines for winter and summer. In summer, for certain cargo ships, the freeboard can sometimes be measured in centiliters. I remember being able to kneel on deck and reach the water with my hand. In heavy seas, the waves are constantly on the deck and the ship can handle it fine; you just don't go there.

Ships often look deceptive about their draft, because you almost never see a ship truly empty. Even when not carrying load, they have a lot of ballast.