Yes, well, the guy has been lifted out of the water in the meantime
Two more tests. I found that the sky was a bit dark when viewed from underwater, and really don't know if that's realistic. I was hoping to get strong local light, and darkness and reflections further away, but didn't succeed so far. So in number one I lighted up the sky. Even tried a white surface shader on the water with a distance shader, which has an interesting effect, but not realistic.
In the second one I tried my way of getting refracted light onto the lake bottom and fish (thin, but thick cloud and 2 suns; one blue, one yellow), but as the water area is so close to the fish and camera, that's kind of hard. The cloud (though only secondary) is reflected (secondary) in the water, as you can see. If it's above, it'll do shadows
on the water, which isn't what is needed either.
Tried a plane with a opacity shadow mask (voronoi warped), but even with no opacity it darkens the whole lot. And without the plane doing shadows it has no meaning.
So, question; is there any way to make a mask (slighly under or above water) to get these patches of shadow across everything underwater? Any ideas are welcome.
Oh, made a quick water boatman to venture across the murk.