How to apply effects depending on a mask? (under-water transition)

I would like to efficiently apply different post processing effects to screen regions that are above and below the water surface.

The following image is what I'm trying to achieve.


The next image represents a mask texture that is updated at run-time, which represents what pixel is above and below the water surface. Black=Above, White=Below.


What I'm looking for is:

• I would like to have anti-aliasing on the entire screen, like shared settings between the two areas.
  
• I want to have different fog, color grading, etc... depending whether the pixel is below or above the water surface.

How can I achieve this efficiently with the Post Processing Stack v2?

 

Sorry for impudence folks but [BUMP] since it's a really needed feature. There was one asset that could do that (UWS) but it's abandoned.

So, again, BUMP.

 

There is a shader that does this..... seems to be managed directly by the shader and a dummy box. don't know. But would be also a elegant solution, instead of a hard cutted edge....

https://assetstore.unity.com/packages/vfx/shaders/dcg-sidescroller-water-shader-68555

But I am not a programmer, nor I am able to edit shaders. But this solution would be something for the crest system =) (found this issue from there)

 

For anyone who lands here, ignore this advice. I purchased this shader and it's just a mesh half a cube. No special cut out being done here. Also you get less than what you see in the video. And no refunds or support from developer.