All I can find is a dumb rule that it's due to the distinction between fresh and salt water bodies. Although if that's the case, why not refer to salt lakes as small seas? I think it should be based on size and not content.
the rabbit hole you need to go down is in linguistics not ecology.
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 word
lake comes from Middle English
lake ('lake, pond, waterway'), from Old English
lacu ('pond, pool, stream'), from Proto-Germanic *
lakō ('pond, ditch, slow moving stream'), from the Proto-Indo-European root *
leǵ- ('to leak, drain'). Cognates include Dutch
laak ('lake, pond, ditch'), Middle Low German
lāke ('water pooled in a riverbed, puddle') German
Lache ('pool, puddle'), and Icelandic
lækur ('slow flowing stream'). Also related are the English words
leak and
leach.