out of one's depth

/ˌaʊtəv wˈɒnz dˈɛpθ/

/ˌaʊɾəv wˈʌnz dˈɛpθ/

out of one's depth

in a very nervous or frustrating state of mind in face of a problem or situation one knows very little or nothing about

in very deep water that might get someone drowned