A: I won't select computer science this semester.
B: Why not?
A: It seems to be out of my depth.
be out of one's depth
Be out of one's depth的意思是“超出某人的理解能力或力所不及”。
E.g. When they start talking about economics, I'm out of my depth.
E.g. The student was out of his depth in discussion on the Middle Ages.
E.g. I'm out of my depth when it comes nuclear physics.
E.g. He's quite out of his depth, you must take him on as your apprentice.