Given that you want to place a water plane at some height in the Poser scene, you can set up a shader on your infinite light that will attenuate the light based on light frequency (color) as the light descends below the surface.

Set up your infinite (sun) like this.

The key parameters are identified with boxes and arrows:

 

Transmission Attenuation Color - This is where you get to say what happens to white light as it descends through the water. Water attenuates red faster then green or blue so the red component should be lowest. Depending on the impurities also absorbing and scattering certain colors, the green can be more or less than the blue. You decide. In this case, I put RGB 160, 190, 200. I use gamma correction so those turn into RGB coefficients according to the rules of gamma correction. If you don't use GC, you may need to adjust this color.

Transmission Distance - This is the distance at which white light turns to the color you gave above. I set the shader up for inches, so this is saying white light becomes the Attenuation Color at 1200 inches. (100 feet) If you decrease the number, light will diminish more quickly. If you increase it, it will stay stronger deeper.

Water Level - This is the level of your virtual water surface, in inches. I have it here set to 2400 inches (240 feet). 

So - in this setup, the "ground" is 240 feet underwater and that is where my bottom is.

 

 

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