noun
Refers to the average height of the sea's surface, used as a reference point for measuring elevation and depth.
その島の最高地点は海水面からわずか2メートルだ。
The highest point of the island is only 2 meters above sea level.
標高 means 'elevation above sea level', often used in geography. 海水面 is the reference plane itself.
Compound of 海 (sea) + 水面 (water surface). The reading かいすいめん is a straightforward combination of the on'yomi readings of the kanji.