Is the water mesh a giant plane, thus probably only 4 verts for each corner? What ‘lighting type’ is the water material?
I don’t remember if the old lighting models for translucency(per vertex, directional, etc) can be fogged or not, but perhaps the default vertex fogging isn’t working for you because the mesh isn’t triangulated enough or the per-pixel fogging isn’t enabled. Either option should work, just depends on where you want to put the cost.
Do you have an Atmospheric Fog actor in your scene too btw? I know awhile back translucency was never fogged correctly with both fog actors. I haven’t used the atmospheric one in awhile to know for sure with the latest versions.