at sea | all at sea

Meaning: If you're at sea, or all at sea, you're confused about something and not sure what to do.

For example:

• I'm all at sea with our new spreadsheet software. I just can't understand it.
  
• For the first few days in her new job, Gail felt totally at sea. She didn't know what to do or who to ask for help.
  

Note: The idiom "all at sea" is used more in British and Australian English, though "at sea" is also sometimes used by speakers of British and Australian English, as well as by speakers of American English.