at sea

1. 在大海上
   He spent 3 months at sea.

   他在海上航行了三个月。

2. 茫然
   I'm rather at sea. Could you explain that again?

   我实在不太明白，你能再解释一下吗？

beyond the sea(s)

1. 在海外
   For many years he had lived beyond the seas.

   他曾在海外住了多年。

by sea

1. 由海路
   He sent his heavy boxes by sea.

   他将那沉重的箱子海运。

go to sea

1. 当水手
   When he was a boy, his greatest wish was to go to sea.

   他小时候最大的愿望就是去当水手。

half seas over

1. 酒醉
   Within an hour he was half seas over and incapable of getting home by himself.

   不到一个小时他就醉了，没法自己回家。

on the sea

1. 在海边
   The city is on the sea.

   这个城市位于海边。

put to sea

1. 出航
   The ship was unable to put to sea on account of the gale.

   由于大风船未能出航。

• at sea
  
  1. sailing on the sea.
  2. (also all at sea) confused; uncertain.
• put (out) to sea
  leave land on a voyage.
• one's sea legs
  one's ability to keep one's balance and not feel seasick on board a moving ship.