also: かいしょう
noun, noun which may take the genitive case particle 'no'
on the sea; maritime; surface of the sea
Refers to the sea surface or to things happening on the sea, often in contrast with land or air. Common in compounds like 海上交通 (maritime traffic) and 海上自衛隊 (Maritime Self-Defense Force).
海上は穏やかで、船は順調に進んだ。
The sea was calm and the ship sailed smoothly.
海上保安庁
The Japan Coast Guard headed to the rescue.