A large body of salt water — the sea or ocean. A basic word often used with coast, navigation, fishing, etc.
夏になると海へ泳ぎに行く人が増える。
In summer, more people go to the sea to swim.
海の向こうに島が見えた。
I saw an island across the sea.
海の生き物についての本を図書館で借りた。
I borrowed a book about sea creatures from the library.