I don't think anything I've done in the Breaker thread will be of any use here. I opted out of using opacity to remove the edges(which is maybe what Richard is meaning, as I mentioned it somewhere there, I think). I was going to try to apply opacity values to remove the square edges of the plane. Instead, I just used a round lake object.
I can tell you, however, that in your clipfile, Ulco, un-checking 'visible to other rays' in the 'ice under water' node fixes the opacity problem. But if you wanted to add translucency/specular/etc. to that layer, I think that may not work as well as you'd need, as those elements wouldn't be seen, would they?
I think there's a better way than this, though. Having a play with it just now, I'll come back if I find anything...