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——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, "freshwater"
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