My head canon for sea-based Kaiju is they have a sack of muscles somewhere inside their body that can expand a cavity, kind of like the diaphragm expands the lungs, except instead of taking in air or water it just creates a volume of vacuum inside of them. This makes them extremely bouyant relative to the surrounding sea pressure, so they rapidly ascend and can casually float like a boat near the surface.
But if they ever want to dive again, they just let that cavity collapse and all their bouyancy goes away.
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.
I mean he clearly just walks out of the water in the movies, he’s not suddenly coming up, like you clearly see him slowly get higher and higher out of the water as he gets closer to shore. Also the ocean isn’t that deep close to shore.
Everyone always thinks the ocean is super deep from the beach outward. This is not the case. Just because a boat will float seemingly close to shore doesnt mean the hull is way down in the water
An aircraft carrier only has a draft of some 12 meters. Godzilla is over 100m. Much of the North Sea is only like 40m deep, with shallows much less than that.