seaport (n.)

Middle English se, seo, from 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, Dutch zee, German See, Swedish sjö), of unknown origin, outside connections "wholly doubtful" [Buck], and an IE etymon "has generally been doubted" [Boutkan]. The meaning "any great mass or large quantity" (of anything) is from c. 1200.

Germanic languages also use the more general Indo-European word (represented by English mere (n.1)) but have no firm distinction between "sea" and "lake," either large or small, by inland or open, salt or 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. The range in the Old English word included "the expanse of salt water that covers much of the world" to individual great, distinctly limited bodies of water; it also was used of inland seas, bogs, lakes, rivers, and the Bristol Channel.

The meaning "dark area of the moon's surface" is attested from 1660s (see mare (n.2)); before the invention of telescopes they were supposed to be water. The phrase sea change "transformation," literally "a change wrought by the sea," is attested from 1610, first in Shakespeare ("The Tempest," I.ii). Sea legs, humorous colloquial term implying ability to walk on a ship's deck when she is pitching or rolling is from 1712. At sea in the figurative sense of "perplexed" is attested from 1768, from literal sense (in reference to ships) of "out of sight of land" (c. 1300).

The sea, the most intact and ancient thing on the globe.

    Everything it touches is a ruin; everything it abandons is new.

[Paul Valéry, "Notebook" entry, 1921, transl. Nathaniel Brudavsky-Brody]

"a bay, cove, inlet, or recess of a large body of water where vessels can load and unload and find shelter from storms; a harbor, whether natural or artificial," Old English port "a port, harbor, a place where there is a constant resort of vessels for the purpose of loading and unloading;" also "a town, market town, city," reinforced by Old French port "harbor, port; mountain pass." The Old English and Old French words both are from Latin portus "a port, harbor," figuratively "haven, place of refuge, asylum" (in Old Latin also "a house;" in Late Latin also "a warehouse"), originally "an entrance, a passage," akin to porta "a city gate, a gate, a door" (from PIE *prtu- "a going, a passage," suffixed form of root *per- (2) "to lead, pass over").

[I]in law, a place where persons and merchandise are allowed to pass into and out of the realm and at which customs officers are stationed for the purpose of inspecting or appraising imported goods. In this sense a port may exist on the frontier, where the foreign communication is by land. [Century Dictionary]

The figurative sense "place, position, or condition of refuge" is attested in English from early 15c.; phrase any port in a storm, indicating "any refuge is welcomed in adversity," is by 1749. A port of call (1810) is one paid a scheduled visit by a vessel in the course of its voyage. The verb meaning "to carry or bring into a port" is by 1610s.