at sea 有三个意思：“在海上（航行）”spend three months at sea 在海上航行了三个月，还有“茫然不知所措”和“弄错”的意思，如：You are altogether at sea in your guessing.你猜得完全离谱了。on the sea就是在海的表面上。
on the sea
a.在海上(在海面上)
例句与用法:
1.The sun glared on the sea.
太阳在海上发出令人目眩的光芒.
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.
神翠鸟一种神话中的鸟,人们认为它即是翡翠鸟,据说当它在冬至期间在海上筑巢时,具有平息风浪的力量
3.There are thousands of awnings on the sea beach.
沙滩上有许多遮阳篷.
4.The mine finally came to rest on the sea bed.
那水雷最后沉入海底.
at sea
ad.离开陆地(茫然)
例句与用法:
1.He felt completely at sea in his new school.
他在新学校中感到茫然不知所措.
2.The refugees were at sea for forty days before reaching land.
这些难民在海上漂泊了40天后,他们的船才靠岸.
3.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.
4.This painting is a representation of a storm at sea.
这幅画表现的是海上风暴.
5.The ship has spent three months at sea.
轮船已在海上度过了三个月.
6.I&39;m all at sea.
我一点主意也没有.
