Here is the idea:
1. Place 2 planes: A - bottom, B - water surface - it has to be 3d water (not sure if bump map works)
2. Apply dark, diffuse material to bottom
3. Apply water material to water surface, with caustics enabled, nest this material in a rayswitch so that it is visible ONLY to GI
4. Place sun so that it shines directly from above (in zenith)
5. Switch engine to VCM
6. Set resolution to square
7. BAM! You end up with nice caustics map
8. Open it in PS, desaturate, inverse colors, save
9. Slightly modify the scene: change bottom color to some lighter one, disable caustics for water
10. Change engine back to standard progressive
11. Create a 3rd plane, between water and bottom
12. Apply pure black material with opacity map to the 3rd plane - as opacity map use (surprise) the caustics map we saved earlier
13. Nest the pure black material in rayswitch, make it visible ONLY to GI rays
14. Render
With SSS you can even turn those into VOLUMETRIC CAUSTICS.
I know this has a lot of flaws and probably can't be used for animations, but this idea was in my mind for some time now. Maybe someone will find a use for it, who knows... :)