Some people believe that gravity doesn't exist

[Gawdzilla Sama]

Not really, because the buoyancy tanks are closed off from the outside. The water in the tanks is the same as the equipment or the crew when it comes to buoyancy.

The buoyancy tank no longer displace water.  It accommodate water now.

You get the concept wrong. The displacement is the hull, not things inside it. The tanks weigh one figure empty and another full, but they don't a change the volume of the submarine by going from one to the other.

---

That depends on how your perceive shape.  If your perception of shape is largely informed by photographs of the outside of the sub, then perhaps it is natural to perceive the shape as having not changed.   If you perceive shape as the geometry of that part of the submarine from which water is excluded, then the shape has manifestly changed.

Submariners perceive shape in a fashion where they don't die on the bottom.