At sea

the salt waters that cover the greater part of the earth’s surface.
a division of these waters, of considerable extent, more or less definitely marked off by land boundaries:
the North Sea.
one of the ; ocean.
a large lake or landlocked body of water.
the degree or amount of turbulence of the ocean or other body of water, as caused by the wind.
the waves.
a large wave:
The heavy seas almost drowned us.
a widely extended, copious, or overwhelming quantity:
a sea of faces; a sea of troubles.
the work, travel, and shipboard life of a sailor:
The sea is a hard life but a rewarding one.
Astronomy, 3 .
of, relating to, or adapted for use at sea.
at sea,

on the ocean.
perplexed; uncertain:
completely at sea as to how to answer the question.

Also, asea.
follow the sea, to pursue a nautical career:
Many boys then dreamed of following the sea.
go to sea,

to set out on a voyage.
to embark on a nautical career.

half seas over, Slang. partly or completely drunk:
They came home at dawn, looking half seas over.
Also, half-seas over.
put to sea, to embark on a sea voyage:
The expedition is nearly ready to put to sea.
Also, put out to sea.
noun

the sea, the mass of salt water on the earth’s surface as differentiated from the land related adjectives marine maritime thalassic
(as modifier): sea air

(capital when part of place name)

one of the smaller areas of ocean: the Irish Sea
a large inland area of water: the Caspian Sea

turbulence or swell, esp of considerable size: heavy seas
(capital when part of a name) (astronomy) any of many huge dry plains on the surface of the moon See also mare2
anything resembling the sea in size or apparent limitlessness
the life or career of a sailor (esp in the phrase follow the sea)
at sea

on the ocean
in a state of confusion

go to sea, to become a sailor
put to sea, put out to sea, to embark on a sea voyage
n.

Old English sæ “sheet of water, sea, lake, pool,” from Proto-Germanic *saiwaz (cf. Old Saxon seo, Old Frisian se, Middle Dutch see, Swedish sjö), of unknown origin, outside connections “wholly doubtful” [Buck]. Meaning “large quantity” (of anything) is from c.1200. Meaning “dark area of the moon’s surface” is attested from 1660s (see mare (n.2)).

Germanic languages also use the general Indo-European word (represented by English mere (n.)), but have no firm distinction between “sea” and “lake,” either by size, by inland or open, or by salt vs. fresh. This may reflect the Baltic geography where the languages are thought to have originated. The two words are used more or less interchangeably in Germanic, and exist in opposite senses (e.g. Gothic saiws “lake,” marei “sea;” but Dutch zee “sea,” meer “lake”). Cf. also Old Norse sær “sea,” but Danish sø, usually “lake” but “sea” in phrases. German See is “sea” (fem.) or “lake” (masc.). The single Old English word sæ glosses Latin mare, aequor, pontus, pelagus, and marmor.

Phrase sea change “transformation” is attested from 1610, first in Shakespeare (“The Tempest,” I.ii). Sea anemone is from 1742; sea legs is from 1712; sea level from 1806; sea urchin from 1590s. At sea in the figurative sense of “perplexed” is attested from 1768, from literal sense of “out of sight of land” (c.1300).
sea
(sē)

The continuous body of salt water that covers most of the Earth’s surface.

A region of water within an ocean and partly enclosed by land, such as the North Sea. See Note at ocean.

A large body of either fresh or salt water that is completely enclosed by land, such as the Caspian Sea.

Astronomy A mare.

Seattle Mariners
Seattle Seahawks
Seattle-Tacoma International Airport
solar elevation angle
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 ]
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 ]
In addition to the idiom beginning with
sea
also see:

at sea
between a rock and a hard place (devil and the deep blue sea)
high seas
not the only fish in the sea
put out (to sea)