Is the river deep enough for swimming (in)?

句末的介词in可以省略。句子意思是：在这条河里游泳水够深吗?

如果不省略介词in，则swimming 可理解为动名词，即swimming in (the river) 为一个动名词短语。

如果不省略介词in，则swimming 可理解为名词（即动名词被名词化）。

如果将句末的 for swimming 改为 to swim，则最好有介词in，即：

Is the river deep enough to swim in? 在这条河里游泳水够深吗?