The stainless steel balls are the pressure vessel. The fiberglass skin covers the skeleton and external equipment only.
The Navy isn’t stupid ... unlike the people who built a submarine out of carbon fiber ...
I worked on the DSRV 2 yrs and it was so antiquated I was stunned. Just something like a 1960’s Indy Car, all manuel switches,old control knobs.Im sure there was A computer but I never saw it while sitting in the Pilots seat. Submariners are a unique bunch and that’s putting it mildly !