noun
upper deck
Nautical term referring to the highest continuous deck of a ship, exposed to the open air.
乗客は上甲板で夕日を眺めた。
The passengers watched the sunset from the upper deck.
甲板 is a general term for 'deck', while 上甲板 specifically means 'upper deck'.
Compound of 上 (じょう, 'upper') and 甲板 (かんぱん, 'deck').