lighting variations inside water

Hi,
I am Implementing a Marine World. Player will be inside the Water. Can you guys help me out on how to implement the lighting variations inside the water, means the simulating the sunlight effect inside the water. I saw it other Aquarium implementations.

I am done with rest of the functionalities and waiting for this feature to finish. If anyone can give me sample source code on how to control or vary the sunlight falling inside the water, It will be great.

Can you just use the caustic pattern as a cookie on your light?