Help: Split underwater/above water view with displacement which also works in orthographic

I want to fill the space below the water plane with a blue color:

I tried doing this with post processing, but the first problem is, that my water has displacement, like this:

There is a forum post with a solutionfor that, but I was not able to figure out how to reconstruct it for my displacement.
I recreated another solution . This one works in 3D View (without the displacement), but not in orthographic.

Anyone an idea how to solve these problems?