水本身不鹹
最貼近核心畫面的一支：直接在講那攤水是哪一種。
adjective · B1
Used about lakes, rivers, pools and the water in them: holding water that has no salt in it, unlike the sea.
淡水的
核心畫面
One way to hold this word: two kinds of water in the world, and this is the one without salt. Rain falls, rivers run, lakes sit still — all of it fresh, all of it drinkable if it is clean. Then the sea, which is the other kind. Everything the word does grows out of that one split. It can describe the water, or what lives in the water, or the water itself as a thing you can measure and run out of.
freshwater 的核心畫面可以想成：世界上的水分成兩種，而這是沒有鹽的那一種。雨水落下、河水流動、湖水靜著——…更多收起
freshwater 的核心畫面可以想成：世界上的水分成兩種，而這是沒有鹽的那一種。雨水落下、河水流動、湖水靜著——這些都是淡水，乾淨的話就能喝。另一種是海水。這個字後來長出來的所有用法，都可以看成從這條分界線走出去：有時候形容水，有時候形容住在那種水裡的生物，有時候乾脆就當成一種可以計量、也可能用完的資源。From the word’s history
From Middle English freche watur, fresshe water; the word is simply fresh plus water, where fresh carries its older sense of not salty.
來自中古英文 freche watur、fresshe water，就是 fresh 加 water。這裡的 fresh 是它比較老的一個意思：不鹹。所以這個字從一開始就只是在講「不鹹的水」，跟「新鮮」無關。
Wiktionary etymology in the supplied packet (kaikki:3f6f69a190036ae4:264638743): From Middle English freche watur, fresshe water; equivalent to fresh + water.
Three branches
住在淡水裡的
畫面沒變，只是鏡頭從水移到水裡的東西：說牠屬於這一種水，不屬於海。
adjective · B1
Used about animals and plants: living in lakes, rivers or pools rather than in the sea.
淡水（生物）的
只在平靜水面上待過的人
這一支把水的對比搬到人身上：淡水是平靜的，外海不是，所以「只跑過淡水」等於「沒被考驗過」。這是比喻，不是另一條字源。
adjective · C2
In older sea writing, used about a sailor who has no real skill or experience at sea, as though they had only ever sailed on calm rivers and lakes.
（舊時航海用語）沒見過大海的、外行的