Water from the sea; saltwater that composes the ocean or sea.
海水は飲めないので、遭難時は注意が必要だ。
You can't drink seawater, so care is needed in case of being stranded.
海水の温度が上がると、生態系に影響が出る。
When sea water temperatures rise, the ecosystem is affected.
海水浴をする前に潮の流れを確認した。
I checked the currents before going for a swim in the sea.