For an underwater scene, real caustics aren't necessary. Because the source of the caustics (the surface waves) are so far from the objects underneath, nobody is going to notice if they aren't exactly correlated. So it's enough to just make the light *look* like caustics.

Here is a screen shot of what I did. I used a cellular node attached to my main sun light. This is attached to an infinite light, pointing close to straight down. I also used an IBL - will post that next.

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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)