out of one's depth
be out of your depth
(British English) to be in water that is too deep to stand in with your head above water
If you can't swim, don't go out of your depth.
I don't like going out of my depth in the sea.
to be unable to understand something because it is too difficult; to be in a situation that you cannot control
He felt totally out of his depth in his new job.
The writer seems a little out of her depth when dealing with the emotional issue involved.
e.g.
I'd be even more out of my depth as a hunter. （狩猟なんて）それこそ素人に出来るようなもんじゃねえだろ (/yupeco/『ゴールデンカムイ』1話)