"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."

I totally agree. In fact I'd go further than that - even if you're looking at a scene from above the waterline and seeing caustics through the water to a close-to-surface bottom, still nobody except a very diligent (and probably nnlly-retentive) scientific optics specialist would notice any anomaly..
As long as the caustics seem in scale, that is.

Cheers,
Diolma

(Long bookmark, but I tend to do that)..

Oh, and thx. bagginsbill, for posting that!