Because of rocks that protrude into the river at Lepenski Vir,

In order for E to be correct, we need something that is "as opposed to" either "whirling water that makes seeing fish easier" or simply "easier."

"As opposed to" does not mean "unlike." It means something along the lines of "rather than" or "not."

It does not make sense to say that the rocks are causing whirling water rather than the rushing waters of the river where it goes through the gorge. Clearly, the point to be made is not that rocks in one area are not causing the rushing waters of another area.

The following would make sense:

The whirling water makes seeing fish easier, rather than harder.

The whirling water makes seeing fish easier, not harder.

The whirling water makes seeing fish easier, as opposed to harder.

The rocks cause whirling water, rather than rushing water.

The rocks cause whirling water, not rushing water.

The rocks cause whirling water, as opposed to rushing water.