Some people believe that gravity doesn't exist

[Anomalocaris]

That’s one way to look at it.

Another way is when the submarine is floating, it’s water displacing shape is the usual cigar shaped cylinder that exclude water from its outside.  When it submerges, it’s water displacing shape becomes a complicated thing in which various parts of the interior of cylinder  transforms into pinched off pockets of the exterior water.

So the submarine changed its water displacing shape, even if not its hydrodynamic shape, when it submerges.

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.