at sea 有三個意思：“在海上（航行）”spend three months at sea 在海上航行了三個月，還有“茫然不知所措”和“弄錯”的意思，如：You are altogether at sea in your guessing.你猜得完全離譜了。on the sea就是在海的表面上。

on the sea

a.在海上(在海面上)

例句與用法:

1.The sun glared on the sea.

太陽在海上發出令人目眩的光芒.

2.A fabled bird,identified with the kingfisher,that was supposed to have had the power to calm the wind and the waves while it nested on the sea during the winter solstice.

神翠鳥一種神話中的鳥,人們認為它即是翡翠鳥,據說當它在冬至期間在海上筑巢時,具有息風浪的力量

3.There are thousands of awnings on the sea beach.

沙灘上有許多遮陽篷.

4.The mine finally came to rest on the sea bed.

那水雷最后沉入海底.

at sea

ad.離開陸地(茫然)

例句與用法:

1.He felt completely at sea in his new school.

他在新學校中感到茫然不知所措.

2.The refugees were at sea for forty days before reaching land.

這些難民在海上漂泊了40天后,他們的船才靠岸.

3.After three days at sea we sighted the land.

在海上航行了三天,我們終于看見了大陸.

4.This painting is a representation of a storm at sea.

這幅畫表現的是海上風暴.

5.The ship has spent three months at sea.

輪船已在海上度過了三個月.

6.I'm all at sea.

我一點主意也沒有.