White paint could work, but the advantage of these shade balls is doesn't need a structure built to be painted on, and it can be scaled up and installed easily by just flooding it on the bodies of water like a ball pit. While mirrors can reflect almost all the light its a bit more expensive compared to the plastic used in shade ball, and there's also factors like the mirror losing its reflectiveness overtime.I think Ocean is to large, maybe minor seas like bohai sea or baltic seas, and as you said not to much of it, since aquatic life needs light.
Also White paint is good, but actual maybe cheap mirrors reflect like 99.9% i guess.
In the same way we can cover chinas inland reservoirs, for example those of the north south water transfer project with it to reduce evaporation of water and at the same time reflect.
There is also idea to use concentrated solar power mirrors to heat sea water to desalinate which at least uses the heat of the sun for industrial purposes.
Put some of those im Gobi and built a canal so that sea water from Bohai enters the dome and voila, fresh water.
And from the remaining Brine China can exteact Lithium & Uranium.
Another good location to flood these would be on the Artic ocean where there's not a lot of human activity or wildlife there that can be disrupted. But I'm starting wonder if these sea spaces should be utilized more better, like having solar panels on top of the bodies of water.
Desalination is another topic I'm interested in as well. There's a lot of possible industrial chemical uses for brine, that I hope will become scalable and help make desalination more cheaper and possibly profitable in the future.