Dic: out of one's depth, beyond one's depth

out of one's depth:

Fig. involved in something that is beyond one's capabilities. (*Typically: be ~; get ~.)

• You know, you are really out of your depth in this project.
• I am sure I am out of my depth in organic chemistry.

beyond one's depth :

1. Lit. in water that is too deep.

• Sally swam out from the beach until she was beyond her depth.
• Jane swam out to get her even though it was beyond her depth, too.

2. Fig. beyond one's understanding or capabilities.

• I'm beyond my depth in calculus class.
• Poor John was involved in a problem that was really beyond his depth.

... McGraw-Hill Dictionary of American Idioms and Phrasal Verbs,

http://idioms.thefreedictionary.com/depth