The best succinct definition for neutral buoyancy I could find was this one, from Wikipedia:
> ...a condition in which a physical body's average density is equal to the density of the fluid in which it is immersed.[0]
This means that once the pod has been weighted to be neutrally buoyant it will be so at any depth of water, since neither it or water are compressible[1] and thus their average densities are unchanged by depth.
The challenges of maintaining neutral buoyancy arise when you submerge a compressible object, such as a scuba diver wearing an inflatable buoyancy control device (BCD). The water pressure (caused by the weight of the water above) will squeeze them and their equipment, compressing anything that is air filled.
To sink from the surface, the diver has to deflate their BCD, and then put sufficient air in it to achieve neutral buoyancy at their desired depth. If they swim downwards, the higher water pressure will squeeze them some more, the air in their BCD will compress some more, and their average density will increase. They will now be negatively buoyant and will sink.
The deeper they sink, the higher the water pressure will be, the more the air in their BCD will compress and the more negatively buoyant they will become. So they'll sink faster and faster. This happened to me once when I was first learning to scuba dive. It can be scary.
The opposite applies to ascending. This is why good buoyancy control is such an important (and difficult) thing for scuba divers to learn.[2]
From a buoyancy perspective, a metal pod is more like a camera in an underwater housing. These are rigid in construction (usually either plastic or metal) and their buoyancy characteristics don't change with depth.
[0] https://en.wikipedia.org/wiki/Neutral_buoyancy
[1] Not strictly true for water, but true enough for scuba diving depths