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.
If we are now considering philosophical intellectual exercises to be memes then this description is accurate.