The water was only knee-deep.

水只有齐膝深。

...knee-deep snow.

齐膝深的雪

They spent much of their time knee-deep in mud. [+ in]

他们在齐膝深的泥地里费了好长时间。

They sometimes sank knee-deep into the marsh.

他们不时陷入没膝的湿地。