at sea，by sea, on the sea, by the sea 的区别

(1) at sea表示“在茫茫大海上”，意指“离海岸很远”。如:  

① Now his ships were all at sea. 现在他的船都出海了。

② The ship hit an iceberg and buried at sea.这船撞上了冰山，葬身海底。

③ Several ships were lost at sea owing to the hurricane. 由于飓风，好几艘船在海上失踪。

④These sailors were at sea for forty days before reaching land.   这些水手在海上漂泊了40天后，他们的船才靠岸。

 ⑤ After three days at sea we sighted the land.   在海上航行了三天，我们终于看见了大陆。

⑥This painting is a representation of a storm at sea.   这幅画表现的是海上风暴。

⑦ The ship has spent three months at sea.   轮船已在海上度过了三个月。

  (2) at sea前面加上all或completely时，引申为“茫然不知所措”之意。如: 

①     I'm all at sea. I can't understand that problem.我简直是一片茫然，我无法理解这个问题。

②     He was all at sea when he began his new job. 他开始新工作时，茫然不知所措。

③     Would you please explain it again? I'm all at sea.你能否再解释一下?我一点儿也不懂。

④     The girl was completely at sea when her mother scolded her. 当母亲责怪她时，女孩茫然不知所措。

⑤     He felt completely at sea in his new school.   他在新学校中感到茫然不知所措。

⑥     I'm all at sea.   我一点主意也没有。

（3） 请注意by sea与by the sea的区别，前者表示“经海路”，后者表示 “在海边”。如:

by sea表示“经海路”；

by the sea表示 “在海边”

① "How did you go there? By train?" "No, we went there by sea." “你们怎么到那里去的?乘火车?”“不，我们乘船去的那里。”

③I'm terrified of flying I'd rather go by sea.   我害怕坐飞机--我宁愿坐船.

④May I ask if you are fond of traveling by sea?   我可以问一下你是否喜欢海上旅行？

⑤ The Turners lived by the sea. 特纳一家住在海边。

（4）注意go to the sea与go to sea的区别

go to the sea意为“去海滨”(度假或野餐)，go to sea表示“去当水手”。如:

①     Last Sunday, the students in Class Two went to the sea.上星期天，二班的学生去海边玩。

②     "I want to go to sea when I grow up." said the boy to his mother. 这小男孩对母亲说：“我长大了要去当水手。”

（5）on the sea:  a. 在海上(在海面上)

例句与用法:

①The sun glared on the sea.   太阳在海上发出令人目眩的光芒。

②There are thousands of awnings on the sea beach.   沙滩上有许多遮阳篷。

③The mine finally came to rest on the sea bed.   那水雷最后沉入海底.

(6)  in the sea:  a. 在海里