Water draws a line on whatever it touches. Put anything into water and a clean edge appears where the wet part stops and the dry part starts. You can point at that edge, walk along it, paint above it, or watch it move. `Waterline` is the name for that edge. The word says nothing about what the water is touching — a hull, a beach, the rim of an — only that there is a line there and water made it.
Simply water + line. The compound puts its own picture on the surface, which is why it transfers so easily from a ship to a shore to an eyelid.
就是 water 加上 line。這個複合字把自己的畫面直接寫在表面上，所以它才能這麼輕鬆地從船身搬到岸邊，再搬到眼皮上。
Wiktionary entry for 'waterline' supplied in this packet, source_ref kaikki:3f6f69a190036ae4:1004399623: 'From water + line.'
Two branches
水在船身和岸邊畫出的線
核心畫面最直接的兩個位置：一條畫在船身上，一條畫在陸地上。兩條都會動。
noun · B2
The line along the side of a boat or ship where the surface of the water touches it. It sits higher up the side when the boat is carrying a lot and lower when the boat is empty.
吃水線（船身與水面相接的線）
借到眼睛上的那條濕邊
把同一個畫面縮到很小：眼皮內緣那條一直濕著的窄邊，也是水跟別的東西相接的線。
noun · C1
The wet inner rim of the eyelid, just behind the lashes. People mostly talk about it when they are putting on eyeliner.
眼睛的內眼瞼線（眼線用語）