Oceangate- this submersible uses two hulls. One is a pressure vessel and the other is a free flood. The outer shell helps in the aerodynamics of the sub and is the free flood shell. This free flood shell allows water on both sides so equal pressure is applied to this surface. This outer shell hides the cables, hoses, controllers, and makes it smooth and aerodynamic.
Sunbacker helped to create parts of this outer shell on Cyclops I and the new Cyclops II. Check out their dives and information- you will be amazed.