That's also what I was wondering. Touching something wet, being warmed by the sun, and all that, can't be good for the black plastic.

They don't care that much about heat, they care more about sunlight not reaching the water, which black balls are more effective at.

why would black be better than white?

i remember an article stating that the melting ice at the poles is accelerating global warming because the white of the ice used to reflect the light coming towards the water

The problem is specifically the ultraviolet photons in the sunlight, which photoactivate the reaction between bromine ions and ozone to form bromate [1].

The visible color shouldn't matter as much as strong absorbance or reflectance in the UV band. I would guess that they're using a plastic that absorbs UV, or contains a dye that does so.

1: http://www.ncbi.nlm.nih.gov/pubmed/18290477

Because white reflects, black absorbs. White balls wouldn't reflect all light upwards. To put it another way, if you were in the water, looking at the surface, which would be brighter? White balls or black balls?

makes sense :)

That's what I thought, too. Shiny silver balls would reflect even more light, and look super-cool!

Being balls, they would also reflect that light into the water

Actually white balls probably would be better. What you normally reach with specular reflection is around 80% reflectivity. With diffuse reflection (white colour) you can get into the 99.x% range much more easily.

And they'd sink

Why would they sink?