At sea is an idiomatic and everyday expression.
Meaning : The literal meaning of at sea is being away on a voyage but is also used as an idiomatic expression in everyday English . It is a way of depicting confusion and uncertainty.
If you are at sea, it means that you are lost and bewildered and do not know what to do.
The expression is also used as all at sea.
1. I feel totally at sea when it comes to interpreting graphs and bar charts.
2. Jim was all at sea during the first few days of his new job.
3. Shahid Kapur is completely at sea regarding the future plans that his father has made for the release of his latest film.