I'm supercilious that the submarine would be made of metal and glass on the external.
Most metals are inert in the human body as they parade no surface antigens for the host immune system to respond).
As a result you should not be at risk of attack by any white blood cells or any other component of the immune system because all components of immunity count on on recognizing enemy antigens.
This would therefore reduce antibodies, accompaniment, B-cells, T-cells, etc useless
hope it helps