If together with the batteries, a kind of ballast is added, buoyancy must be increased as well, and this, can only be achieved by adding volume.
If volume is increased while keeping the "single watertight compartment" concept, working pressure (depth) results drastically decreased unless wall thickness grows at the same time.
Some key submersible design starting points, that work one against the others, must be taken into account, and a choice must be made depending on desired features.
From a structural point of view:
Sphere is the best shape, but it is expensive and hard to be built. The second choice is a cylinder, where the balance between thickness, lenght and diameter have to be adjusted.
From the hydrodynamic point of view.
The higher L/B (Lenght/Beam), the better, but at the same time, the less wetted surface the better.
Minimum wetted surface for a given volume, is achieved by a sphere, but its L/B=1, and hence, shape dragg is quite big.
Best L/B is achieved by a very long, very narrow sape, but as a volume is required, beam is limited by practical concerns. Hence a big wetted surface results and surface dragg becomes big as well.
From experimental channel tests, the best practical L/B while minimizing wetted surface, resulted to be between 7 and 8.
At the same time, in a long shaped submersible, the Reynolds number grows very fast along the length, deriving in a quite turbulent flow astern, that badly affects propulsion and yaw-pitch stability.
What means long ? A body submerged in a fluid is short or long, not depending on its size, but on its Reynolds number that relates speed and length to fluid conditions.
For this kind of ROV device, speeds are very little and hence, the capacity for withstanding high presures prevails over dynamic considerations.
Hope to see your modifications to the ROV.