Personally, the only place I'd ever even consider swimming in China would be the ocean. Pools are swam in (and God knows what else) by 150,000 of your closest friends every day; and since pool chemicals cost money and are a lot of inconvenient trouble, we can just do without them, right? Rivers and other fresh water are just a mess...a nightmare of pollution, raw sewage, parasites, and nasty, nasty bacteria. Like Typhoid.
The ocean may not be a lot better, but at least you got some salt on your side there. And enough volume to kinda thin things out a bit.
The beach season can run from nearly all year in places like Sanya in the deepest south, to July-August in Northeastern cities such as Dalian or Qingdao.