在海里游泳的英文表达是 "swimming in the sea"。

这个表达是由两个单词 "swimming" 和 "sea" 组成的。"Swimming" 是动词，表示 "游泳" 的意思；"sea" 是名词，表示 "海洋" 的意思。

下面是一些例句：

I love swimming in the sea. (我喜欢在海里游泳。)
He spends his summer vacation swimming in the sea. (他度假时在海里游泳。)
The water is so clear and clean, it's perfect for swimming in the sea. (水很清澈且干净，非常适合在海里游泳。)