Now when I add a (flat) water plane 200 feet above the ground, and look down from above, I see the depth is darker and bluer than above the water.

Note that I'm using refraction. You were using transparency. Don't use transparency - it's completely fake looking.

The water surface bends the light. What is at the bottom should look flattened.

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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)