Pentarius has a carbon fiber cylindrical pressure vessel with a aluminum hemisphere at one end and a fused quartz hemisphere at the front.
The hull is flooded, and the rest of the components (batteries, motors, and control servos) are oil filled but subjected to full ocean pressure. This saves the weight of creating pressure protection for each of these components.
The pressure at FOD, full ocean depth, is 16,000 PSI. The two hemispheres exert 15,000,000 pounds of compression on the carbon cylinder - the weight of three fully fueled space shuttles!