be all at sea

To be greatly puzzled, perplexed, or completely confused (about something or in some situation). I tried to do well in this class, but I've been all at sea since we started. Bill is all at sea because of this calculus problem. Can you help him? I'm all at sea in this new job of mine. I've never been responsible for these kind of contracts before.

To be bewildered, to have lost one’s way. Presumably it reflects the idea of literally having lost one’s bearings while at sea. It was so used by Dickens and other nineteenth-century writers.