水面
すいめん · suimen · JLPT N2 · noun
surface
🌊 水 (water) + 面 (face/surface): the water's face is its surface
水 is water and 面 means face or surface, so 水面 literally is the face of the water. Think of 面 as the flat 'face' a lake or pool presents to the sky.
水 — pictograph of flowing water
水 is a pictograph depicting a stream of flowing water, with the central line representing the main current and the side strokes showing splashing droplets or ripples. In oracle bone script it was drawn even more clearly as a wavy vertical line with branches. It retains its original meaning of 'water' and also serves as a radical, often appearing in the compressed form 氵.
面 — enclosure 囗 + 自 (nose/face inside)
Originally a pictograph showing a face framed by an outline, with 自 (nose) inside representing the central feature of the face. The early meaning was 'face,' which extended to 'surface,' 'side,' and 'aspect.' In simplified Chinese, 面 also absorbed 麵, adding the meaning 'flour/noodles.'