Pair with its opposite 淡水 (dànshuǐ, 'fresh water') — both are everyday vocabulary for talking about water sources, fish, and ecosystems. 咸水鱼 = saltwater fish, 淡水鱼 = freshwater fish.
memory
咸 (xián) means 'salty' (one of the five basic tastes alongside 甜 sweet, 酸 sour, 苦 bitter, 辣 spicy). The traditional form 鹹 contains 鹵 (lǔ, 'salt'), making the meaning visible — the modern simplified form lost this clue.
