boat people 的用法没问题。其实这是固定搭配（可视为复合名词），在词典上可以查到的。比如《牛津高阶英汉双解词典》就收录了boat people，所给解释是：people who escape from their own country in small boats to try to find safety in another country 船民（乘小船逃到他国的难民）

网友的句子意思是：

So far, 51 boat people have been forcibly repatriated.

到目前为止，已经有 51 名船民被强制遣送回国。

再补充几例：

Today they begin the forced repatriation of Vietnamese boat people. 今天他们开始强制遣返越南船民。

Turning Cuban boat people away would be an inhumane action. 不准这些古巴船民入境将是一种不人道的行为。