海面 (hǎi miàn) refers to the sea surface or to sea level used as a measurement baseline (e.g., "above sea level"). It is more specific than 水面 (shuǐ miàn), which can mean the surface of any body of water (lake, river, etc.). When measuring elevation, 海面 is the fixed reference point, while 海拔 (hǎi bá) is the actual altitude measured from that baseline.