out of one's depth - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Also, beyond one's depth. Outside one's understanding or competence, as in He was out of his depth in that advanced calculus class, or The conductor realized that playing the fugue at the right tempo was beyond their depth. This expression alludes to being in water so deep that one might sink. [c. 1600] Also see over one's head.