水被分为很多水系,水域,所以waters可以表示水域,水系的意思.
举例：This factor plays a significant role in virus decline in coastal waters.
这种因素对沿岸水体中病毒的下降起着重要作用.
Today people are still exploring some of the world's last unknown waters.
现在人们仍然在探索这个世界上一些未知的海域
同样的skys也表示复数.
举例：Nice to see you,if you work hard,skys the limits here.
很高兴见到你,好的开始是成功的一半.
The Heaven and the river are still standing.With the help of wing of memories,I will eventually fly with you,in the sunshine,above the river laying between skys and seas.
天虽未荒,河虽未竭,仍有我,最终会插上思念的翅膀,迎着阳光,在蓝天和蓝海之间的河 流上空,携子之手,飞翔.
Still waters run deep 的意思是静水流深.这里的waters没什么特别的意思就是一个统称和复数的意义.