在某些习语中通常不用冠词。如：

put to sea开航，起航 

go to sea (as sailors) （当水手）出海

be at sea = be on a voyage (as passengers or crew) （作为旅客或船员）乘船航行

除此之外原则上要和冠词。如：

I like to bathe in the sea. 我喜欢在海里游泳。

The sea calmed down. 海水平静下来。

The sea was now calmer. 海比较平静了(www.yygrammar.com)。

My love for you is deeper than the sea. 我对你的爱比海还深。

Aquanauts go to the floor of the sea. 海洋研究人员到海底去探查。

The sea can be blue or green or grey. 海水可以是蓝色、绿色或是灰色。

A damp breeze from the sea usually means rain. 海上吹来的湿风通常意味着要下雨。

The aeroplane out of control and fell into the sea. 飞机失去控制坠入海中。

A man fell overboard into the sea and was drowned. 一名男子从船边坠人海中给淹死了。

There was an icy easterly wind blowing off the sea. 有一股由东边海上吹来的凛冽的寒风。