Just got a reply from e-frontier. I'm going to paste it in verbatim:

Good news and bad news. My Firefly guru tells me that Firefly can't do caustics as we don't have photon mapping- Shade does have this, even the low-end designer LE version, but Poser doesn't. The good news is, you should be able to fake it with a combination of shaders on lights and perhaps atmosphere effects. I'll try to work up a tutorial next week to show how to do decent-looking underwater lighting effects. (My guy mentions by the way that the caustics in Finding Nemo were faked rather than being simulated; I guess if Pixar does it that makes it OK...)

And thanks for providing an interesting question! I love getting this kind of thing 'cause it makes me think creatively.

Regards, Colin Gerbode
Technical Support

So, an answer of sorts is in the works. It's just a matter of time.