not having the knowledge, experience, or skills to deal with a particular subject or situation

I was out of my depth in the advanced class, so I moved to the intermediate class. 由于高级班对我来说太难，我转到了中级班。

in water that is so deep that it goes over your head when you are standing

I'm not a strong swimmer, so I prefer not to go out of my depth. 我不谙水性，所以我不会进入水深超过我身高的区域。