Nice image and render so far.
I think the reason you cannot see the strength of the sunlight down into the medium of water is from two things.
(1) Because Maxwell Render is physical render engine, you water must be modeled just as it would be in physical space. So if this is a box container with a volume of water. Then this is how maxwell render thinks when looking at you scene.
Maxwell see light hit the water plane surface and calculates it according to your settings for that dielectric, but for light to bend, or refract it has to hit another surface of some type.
So Maxwell sees the light hit the water plane surface but it never again encounter a second surface that lets it finish the calculation of the light transmission through the water because the next surface it hits is your box.
I would add another surface for you water and see how that develops.
(2) I would set your iso much higher and your fstop much higher to increase the strenght of the light transmissions- we may catch more of the type of caustics you are look for.
Hope that helps! I did not want to get more technical on the topic of light interaction with Maxwell because there is very lengthy thread on the concept using a glass and liquid as example that was discussed in the main forum in detail.
ref:
http://www.maxwellrender.com/forum/view ... ass+liquid