watertight door
Technical term used in maritime, engineering, and construction contexts for a door designed to prevent water from passing through.
船の水密扉は、浸水を防ぐために閉めておく必要がある。
The ship's watertight doors must be kept closed to prevent flooding.
Compound of 水密 (watertight) + 扉 (door). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning.