that's probably where i'd start - simply don't include the water There is/was a node for testing depth behind a surface that looks like it's specifically intended for this purpose on (probably) 5.2. I cant remember what it was called but if you search for depth in the materials nodes it should come up
Someone else could explain it more technical terms, but: The unreal water shader isn't actually translucent - its doing a trick where it projects what's underneath onto the water plane. This all happens prior to the point where the outline post process happens (its right there in the name - post process) - the depth data…