When applying image processing post process like vignette over a scene containing the water material, the post process affects the shading of the water material differently than other scene elements.
Notice dark vignette is overly affecting the color of the water at the horizon. It should blend better with the skybox material as it does in the original below, even if there is a vignette overlaid.
Depending on the colors of the picture, some areas will be darkened more than others, as in your picture where the blue of the sea is darker than the blue of the sky.
From the PGs, you can see at the horizon, where the water meets the skybox, without a vignette the water blends well into the skybox as it’s reflecting the white horizon above. But with the vignette, the vignette color of black seems to be tinting the reflected white of the water MORE than the white of the skybox horizon, so that there’s a stark difference between the water and sky (it’s no longer well blended).
The image processing is applied to all the intermediate rendering as well to the final rendering, so you end up having two image processing applied to the water (the intermediate passes are the ones that generate the reflection and refraction textures).
What you need is to apply an image processing a single time, at the very end of the rendering. To do that, you should se a DefaultRenderingPipeline and enable the vignette there:
One caveat here: the water material is applying a convertion to linear space that it should not do, as the reflection/refraction textures are already generated in linear space. The easiest (only) way I found to disable that is to disable the image processing configuration of the water by doing water._imageProcessingConfiguration = null;