This is a very good idea albeit not a new one. But they have implemented it in a significant way in Japan:
"...What is the advantage of a "floating" solar power system design? Kyocera said the cooling effect of the water results in more electricity generated than with ground-mount and rooftop systems. Also, by shading the water, they reduce reservoir water evaporation and algae growth...."
So this is good both for solar energy and water conservation.
If you look at the silent video in that link, you get a vague sense of the huge scale of this floating solar plant.