be at sea

1. To literally sail on the sea in a boat or ship. A: "Is your mother at sea yet?" B: "Yes, her cruise lasts several weeks." How long do you all plan to be at sea? Now that I've got my new boat, I am just itching to be at sea again!

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

at sea, to be/all

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.