1.
horizon, sea horizon
The apparent line where the sea meets the sky when viewed from the coast or at sea. Specifically refers to the horizon over water, as distinct from 地平線 (horizon over land).
Usage
Japanese distinguishes between two types of horizon: 水平線 is the horizon over the sea or ocean, while 地平線 is the horizon over land (plains, desert, etc.). 水平線 literally means "water-level line."