Your sunlight (mostly visible and IR < <a href="http://www.globalwarmingart.com/wiki/Image:Solar_Spectrum_png>)">http://www.globalwarmingart.com/wiki/Image:Solar_Spectrum_png>)</a> traverses at best 0.8cm of water (through the sheet once and then reflected back from the white styrofoam) and can only absorb about 25% or the solar radiation and mostly in the IR. You would get better results by adding dye to your water to turn it practically opaque or by painting the back of y