at sea

1.  Aboard a ship, on the ocean, as in Within a few hours the ship would be out at sea. During World War II a famous American newscaster addressed his radio broadcasts to listeners everywhere, including "all the ships at sea." [1300s]
2.  Also, all at sea. Perplexed, bewildered, as in She was all at sea in these new surroundings. This idiom transfers the condition of a vessel that has lost its bearings to the human mind. Charles Dickens used it in Little Dorrit (1855): "Mrs. Tickit ... was so plainly at sea on this part of the case." [Second half of 1700s]

(DOD) Includes the following maritime areas: foreign internal waters, archipelagic waters, and territorial seas; foreign contiguous zones; foreign exclusive economic zones; the high seas; and US-exclusive economic zone, territorial sea, and internal waters.

The adjective has 2 meanings:

Meaning #1: perplexed by many conflicting situations or statements; filled with bewilderment
  Synonyms: baffled, befuddled, bemused, bewildered, confounded, confused, lost, mazed, mixed-up

Meaning #2: traveling by boat or ship
  Synonym: sailing