Jaggy values on render target, how to interpolate them?

Hey guys, hope you’re all good.

I’m doing a half in / half out underwater post process. Right now we’re using the clip plane projection to see which part of the screen is currently underwater or not, by checking the world position of that pixel versus our world aligned render target height field. This works “as intended”.

As you can see, the problem arises when the resolution or density (because it can have big resolution but on a big ocean patch), is not high enough, the line or separation looks completely jagged.

I’m trying to see if there’s a way to interpolate this central values with the neighbouring piels on the Render Target, but that doesn’t seem to work. Using trilinear filtering for the render target doesn’t.

DO any of you have any idea how i can smooth this edges or height without having to rely on having big resolutions or small patches?