Hippopotamus after evading lion

An impala, an antelope having lyre-shaped horns, is struggling to get away in the muddy water, in the footage taken at the Nsemani Dam near Satara in the Kruger National Park in South Africa on Aug. 4, 2015.

A lion gazes at this out of the muddy water.

Why is the impala struggling in the muddy water? A lone lion is on pursuit.

Which choice is a better one for the impala? Waiting in the muddy water until the lion leaves or going out of the water?

While impala struggles in the water, a hippopotamus comes and cruelly kills the impala.

The lion just stares at this.