Thanks for your quick response. Unfortunately changing the translucency sort priority did not fix my problem.
I dug a little deeper and I think the reason why translucent and additive materials are not rendered underwater (on top of the fog) is the stencil setup of the “M_Underwater_PP” material, used in “PP_Global”. Your stencil video regarding masking the surface (for boats and stuff) did not help me unfortunately, as my problem is not to “cut out” something, but render a translucent or additive material on top of the underwater fog.