Attached Link: http://www.halcyon.com/alrives/brycetips/masters/underwater/main.html

I have seen a few attempts to do caustics in Poser, but I have never found them to be very believable. I learned to do them in Bryce.

There are several good tutorials (I haven't checked here at Rosity, but I would say to look in there):

http://visualmagic.awn.com/archive/archive06.html

The link shows you the one from which I created my image, "The Maremaid".

Hope this helps you.
Cris