Gee, subs have small arms? Just kidding.
I was on a Navy DLG in the early 70s, for a little over 3 years.
Foreign ports had customs inspectors that came aboard and interfaced
with the Captain. There were "papers" signed and sealed to effect the
bonding requirements. As far as I know we never were actually
inspected by anyone other than Navy personnel. Liberty was never
granted until after we cleared customs, including arrival back in the
USA.