海洋
かいよう · kaiyou · JLPT N2 · noun
ocean
🌊 海 = sea (near the coast) + 洋 = vast open water; together they name the whole ocean
海 is the sea close to land, and 洋 is the huge open water beyond it. Joined as 海洋, they cover the entire ocean.
海 — water radical 氵 + 每 (phonetic)
海 is a phono-semantic compound. The 氵 (water) radical signals the meaning—a vast body of water—while 每 supplies the sound. Originally referring to the sea or ocean, it later extended figuratively to mean 'vast' or 'a great quantity,' as in 人海 (a sea of people).
洋 — water radical 氵 + sheep 羊
洋 combines the water radical 氵 (meaning) with 羊 'sheep' (phonetic, hinting at the sound yáng). Originally it referred to a vast body of water or ocean. Later it extended to mean 'foreign' or 'overseas,' since foreign things came across the ocean.