They had their backs to the sea.

意思是：

他们背靠着大海。
他们的身后就是大海。

这里的重点是 have one's back to...

have one's back to...

= 背对着…… / 背靠着……

He had his back to the door.
他背对着门。

She had her back to the window.
她背对着窗户。

所以：

They had their backs to the sea.

拆开：

• They = 他们
• had = 处于……的状态
• their backs = 他们的背
• to the sea = 朝着大海 / 对着大海

直译就是：

“他们的背对着大海。”

自然中文就是：

“他们背靠大海。”

如果是在战争/敦刻尔克这类历史纪录片中，这句话通常还有很强的画面感：军队被逼到海边，身后已经没有退路了。