湖
みずうみ · mizuumi · JLPT N4 · noun
lake
🌊 mizu (water) + umi (sea) = a freshwater sea, i.e. a lake
みずうみ is a native compound: 水 mizu (water) + 海 umi (sea). A lake is literally a water-sea, a sea made of fresh water rather than salt. If you know mizu and umi, you already know this word.
湖 — water 氵 + 胡 (phonetic)
湖 is a phono-semantic compound. The water radical 氵 gives the meaning, indicating something related to water, while 胡 (hú) supplies the sound. Together they denote a lake — a body of standing water — which has been its core meaning since early usage.