有the 无the区别大
有the更强调地点本身。
无the表示抽象。
in the sea，在大海里。
in sea在海上。
比如：in the prison在监狱里
in prison蹲监狱。
in the sea意思是在海上；在海洋里。
短语
In particular the sea 尤其是海
Sinking in to the sea 沉入了海底
man-in-the sea 水下居住人员
例句如下：
We swim in the sea.
我们在这个海里游泳。
I had been on my own for 18 hoursin the sea for 43 hours in total.
我一个人在海上度过了18个小时，从我们出海算起，则是43小时。
The light Ships fared the bestas not so much labouring in the Seabut two or three of them drove, and came close by us, running away with only their Sprit-sail out before the Wind.
小船的境况要算最好了，因为在海上小船容易行驶。但也有两三只小船被风刮得从我们船旁飞驰而过，船上只剩下角帆而向外海飘去。
During the Ordovician, most life was in the sea, so it was sea creatures such as trilobites, brachiopods and graptolites that were drastically reduced in number.
在奥陶纪，大多数生命都生活在海洋里，所以海洋生物，例如三叶虫、腕足类动物与笔石动物的数量在此时急剧减少。
This is highly pertinent. The wind direction, should there be a plume containing radioactivematerial, determines whether fallout will land on populated areas or in the sea.
这是一个关键问题，如果含有放射物质的云团出现，那么风向将决定这些放射性坠尘会降落在聚居地还是海洋。
Those tensions have included a chorus of complaints from Vietnam and the Philippines, which say China has repeatedly interfered with their vessels in the sea.
这些紧张局势包括越南和菲律宾一致抗议中国在海上多次干扰两国船只的航行。
The growth of plankton in the sea is always limited by something. It may be light, or a familiarnutrient such as nitrate or phosphate.
海洋浮游藻类的生长总是受到一些东西的限制，这些东西可能是光，也可能是相似的营养物质比如硝酸盐或者磷酸盐。
in the sea 就是“在海里面”的意思swim in the sea 在海中游泳
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就是在海的表面上比如书The book is on the desk.书在书桌上
