China has built a huge solar array on a lake that used to be an old coal mine, the world's largest floating solar farm. The water helps to keep the panels from overheating.
This article doesn't talk about water quality. A lot of old mine pit lakes are horribly toxic. So the panels might help keep waterbirds from using the lake. But if the water is clean, and if the lake is used by wildlife, I hope they took that into consideration in the design.