So, I have looked at the original Russian text and also at the ruleset how it is implemented.
I propose the following English translation:
STR_OCEAN2_UFOPEDIA: "The ocean is a harsh environment. If the aquanaut's armor is sufficiently damaged, they will take stun damage each turn (only the under armor is relevant for the calculation).{NEWLINE}Stun damage also applies to aquanauts wearing any armor fundamentally not suitable for deep diving (e.g. Aqua-lung)."