The image above is the render of a free mental ray scene available on my website. However, I'll occasionally get an email in regards to a problem with this scene. The reason people email me about this scene is because the caustics don't show up in the render as expected when this scene is rendered with anything higher than Max6. So why is that?
Well, I created that scene waaaaay back in Max6 days. A lot has changed since then, both with 3dsmax & mental ray, and the way I setup scenes now. But specifically there are two reasons the file doesn't work properly with newer versions of 3dsmax:
First, I setup the scene using generic units of measurement. Generally when you work with any form of GI/caustics/FG, you'll be better off using a "real world" unit of measurement (such as feet, inches, mm, cm, etc.)
Second, the way photons were distributed drastically changed from Max6 to Max7. I won't go into all the details as to what changed since it can all be found in the new features guide of Max7...and since the whole deal is rather dated now. But in a nutshell, the photon calculation was made more accurate after Max6. BTW, speaking of the new features guide....please bring it back Autodesk (seriously, pretty please?)
Ok, so how can you make the photons work in this scene with newer versions of Max? The answer is to simply increase the photon energy in this scene until you see the photons. The problem isn't that the photons aren't being generated, it's just they are too dim to be seen. BTW, 100 for the energy level seems to work for this scene in Max9 (and I'm referring to the photon energy setting on the MRspotlight in this scene).
Now, since I'm speaking about water have you tried using the parti-volume shader for water? You might want to give it a try with using a murky mie or hazy mie setup such as this:
Example (click to enlarge)
You can find the murky mie / hazy mie parti-volume settings in the mental ray documentation that ships with 3dsmax.
