YOU SAID:
I can't breathe underwater unless I need to.
INTO JAPANESE
私は必要がない限り水中で呼吸することはできません。
BACK INTO ENGLISH
I can not breathe underwater unless it is necessary.
INTO JAPANESE
それが必要でない限り、私は水中で呼吸できません。
BACK INTO ENGLISH
I can not breathe underwater unless it is necessary.
That didn't even make that much sense in English.