Fog from water is not as close compared to when darkness fully closes in, which means darkness fog will be the only fog prevalent when you are underwater with darkness. However, as soon as you get darkness or while the darkness is fading away, the fog for a brief second is much farther than water fog. However, the water fog would still not be rendered until the darkness effect wears off.

Notice in the screenshot I have no water fog, this is because I took it as soon as I applied darkness to myself.

One obvious solution would be to allow multiple fog effects to be applied simultaneously.