在大海游泳可以用英文表示为 "swimming in the sea" 或 "swimming in the ocean." 这两种表达方式都是指在海洋里游泳。

来源：

"Swimming" 是英语中表示游泳的动词。它来源于中世纪英语 "swimmen," 它本身源自古英语 "swimman," 其中 "swim" 是动词，意思是游泳。

"Sea" 是英语中表示海洋的名词。它来源于古英语 "sæ," 其中 "sæ" 是名词，意思是海洋。

"Ocean" 是英语中表示海洋的名词。它来源于希腊语 "ὠκεανός" (ōkeanós), 其中 "ὠκεανός" 是名词，意思是大海。

下面是一些使用 "swimming in the sea" 和 "swimming in the ocean" 的英文例句，以及它们的中文翻译：

"I love swimming in the sea on hot summer days." (我喜欢在炎热的夏日里在海里游泳。)
"The water in the ocean is so clear and blue." (海洋里的水是如此清澈和蓝色。)
"We spent the day swimming in the ocean and building sandcastles." (我们花了一天在海里游泳和建沙堡。)
"The waves were rough, but we still had fun swimming in the sea." (海浪很大，但我们在海里游泳还是很开心。)