You can picture saltwater as one plain thing first: water with salt in it — the water of the sea. Hold that picture, and everything else is a label peeled off it and stuck onto another word. Stick it on a living thing and you get where that creature belongs. Stick it on a lake or a pool and you get what is inside it. Stick it on a group of people, and you are no longer talking about water at all — only about where they sit on a map.
saltwater 可以先想成一樣很單純的東西：帶鹽的水，也就是海水。先把這個畫面抓住。其他用法都像是從這攤水上撕下一張標籤，再貼到別的字上面。貼在生物身上，講的是牠住在哪一種水裡。貼在湖、池、缸上面，講的是裡面裝的是什麼水。貼到一群人身上，就已經不是在講水了，而是在講他們在地圖上的位置。
From the word’s history
From Middle English saltwater, salte water — literally salt plus water, the same two pieces still visible in the modern spelling.
來自中古英文的 saltwater / salte water，就是 salt（鹽）加 water（水）。今天的拼法裡這兩塊還看得出來，所以「有鹽的水」這個畫面一直沒有離開這個字。
Wiktionary, "saltwater"
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