A very early borrowing into English and said to be the only Latin nautical term used in the Germanic languages (German Anker, Swedish ankar, etc.). The unetymological -ch- emerged late 16c., a pedantic imitation of a corrupt spelling of the Latin word. The figurative sense of "that which gives stability or security" is from late 14c. Meaning "host or presenter of a TV or radio program" is from 1965, short for anchorman (q.v.).
Old English sæ "sheet of water, sea, lake, pool," from Proto-Germanic *saiwa- (source also of 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.
Germanic languages also use the general Indo-European word (represented by English mere (n.1)), 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 (such as Gothic saiws "lake, marshland," marei "sea;" but Dutch zee "sea," meer "lake"). Compare also Old Norse sær "sea," but Danish sø, usually "lake" but "sea" in phrases. German See is "sea" (fem.) or "lake" (masc.).
Boutkan writes that the sea words in Germanic likely were originally "lake," and the older word for "sea" is represented by haff. The single Old English word sæ glosses Latin mare, aequor, pontus, pelagus, and marmor.
Meaning "dark area of the moon's surface" is attested from 1660s (see mare (n.2)). Phrase sea change "transformation," literally "a change wrought by the sea," is attested from 1610, first in Shakespeare ("The Tempest," I.ii). Sea anemone is from 1742; sea legs, humorous colloquial term implying ability to walk on a ship's deck when she is pitching or rolling 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).
Middle English North-se, from Old English norþsæ, norðsæ, usually meaning "the Bristol Channel" (see north + sea). The application to the body of waternow so called, east of England (late 13c.) is from Dutch (Noordzee, Middle Dutch Noortzee); it lies to the north of Holland, where it was contrasted with the inland Zuider Zee, literally "Southern Sea"). To the Danes, it sometimes was Vesterhavet "West Sea." In English, this had been typically called the "German Sea" or "German Ocean," which follows the Roman name for it, Oceanus Germanicus. "German" persisted on some British maps at least into the 1830s. North Sea in Middle English also could mean "the northern portion of the ocean believed to surround the earth" (late 14c.).