seayoudaoicibaDictYouDict[sea 词源字典]
sea: [OE] Sea is a widespread Germanic word, related to German see, Dutch zee, Swedish sjö, and Danish sø (the Scandinavian words are now more usually used for ‘lake’ than ‘sea’). These all point back to a prehistoric Germanic *saiwiz, but it is not known where that came from.
[sea etymology, sea origin, 英语词源]
sea (n.)youdaoicibaDictYouDict
Old English sæ "sheet of water, sea, lake, pool," from Proto-Germanic *saiwaz (cognates: 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 (such as Gothic saiws "lake," 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.). 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).