Here I've made the sky dome bottom half transparent, and now my ray bias on the refract gives me the foamy edge. Of course it's too wide, but I just wanted to show it clearly. Notice the water is casting a shadow. You can choose to turn the shadow on or off. With the shadow on, it darkens the underwater sand nicely. But it isn't necessary. However, since I was doing murky water, this makes sense and produces a realistic effect.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)