Shadows on water are tricky. If the water is crystal clear, the only effect a shadow should have is on the specular reflections from the surface of the water, and the shadow on the bottom as seen THROUGH the water. Poser's specular light will automatically take shadows into account - specular reflection is directly from the light source (sun) to the surface, then to the camera.

As seen in my post #29, shadows from things above the water (like trees or foam) actually show up on the BOTTOM, not the water itself. If you turn off cast shadows for the water plane, then the only shadows on the bottom will be from other things.

A shadow falling on the water should have NO effect on the surface reflections. So this is why you should make sure that the base material has "Reflection_Light_Mult" and "Reflection_Kd_Mult" turned off. Otherwise, Poser multiplies how much to reflect by how much light is hitting that spot, which has nothing to do with reality.

Now if you want to model murky water, then we have a problem. Nothing in the model for water (reflect, refract, or specular) is going to properly render the effect of light and shadow on particles SUSPENDED in the water, between the surface and the bottom.

In the picture above, I tried another technique to address this. I placed 8 squares between the water surface and the bottom. On each of these, I connected a "random" node to a bias node (bias .1) and then that to the transparency inputs of the material. This creates random speckles that interact with light. But most of these squares is transparent. I layered this more or less evenly between the surface and the bottom. The color on them was just ordinary diffuse greenish brown. The result is that each of these catches some of the light or shadow, and lets the rest through to the next layer. As a result, the bottom is indistinct and the shadow from the tree fades nicely as the light finds it harder and harder to reach the bottom.

I think this is pretty realistic.

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