at sea
at sea (1) {adv.} or {adj. phr.}
1. On an ocean voyage; on a journey by ship.
They had first met at sea.
2. Out on the ocean; away from land.
By the second day the ship was well out at sea.
Charles had visited a ship in dock, but he had never been on a ship at sea.
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Source: A Dictionary of American Idioms
