Hah! I'm an idiot. In all that, I forgot to set the IOR on the Refraction. Use 1.33 for water. Doh!

See how the grass from the environment sphere is showing like underwater plants? I love it!

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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)