You may think of a relationship similar to that of 少年/少女, 俳優/女優 or 兄弟/姉妹.
地平線 ／＼地平線水平線
According to the Wikipedia definition, 水平線 is an alternative name of 地平線 used when the surface is watery:
地表面が海などの水面であれば、水平線とも呼ばれる。
In that example, 水平線 is more specific and usually better, but 地平線 is not necessarily wrong, either.
(Also, I think it's technically possible to see a ship coming out from behind an island.)