In researching design options for clients over the years, I've looked into what's involved in building natural swimming pools -- that is, ponds that stay safe enough for swimming without the use of chemical inputs. Here are some of the best links I've found on the subject:
Organic Pools DIY Manual by David Pagan Butler (PDF, 1.2 MB, 2013)
Build Your Own Natural Swimming Hole by Douglas Buege and Vicky Uhland (Grit Magazine, 2007)