From the National Post: Huck Finn Chic: Swimming Ponds.

Natural swimming ponds first appeared in the mid-1980s in Germany and Austria. Since then they have become popular throughout Europe — there are an estimated 50,000 natural ponds — in residential gardens, hotels, spas and parks, where municipalities have introduced them for public swimming. In Germany, the facilities are used year-round: for swimming in summer and for skating in winter. Given Canada’s hot summers and cold winters, the pools seem made for our climate. Over the past three years, natural pools have appeared in the United Kingdom, and there are now a handful of companies and architects who are constructing them in North America.

In fact, the term "natural" swimming pond is a misnomer, since there is nothing natural about them. They are carefully designed and constructed. The water is cleaned by the surrounding plants but the ponds also require pumps and aeration to give nature a little assist. Even so, a well-constructed swimming pond displays no evidence of the plumbing that allows its owner to enjoy all the benefits of a garden pond and its variety of water plants. [continue]

Related

• Taking the plunge naturally ("Imagine having a swimming pool which cleans itself with plants and requires no chemicals. Landscape designer, Michael Littlewood, presents a design that is beautiful, healthy, ecologically diverse and less expensive than conventional pools.") – Permaculture Magazine
• Natural swimming pools – newbuilder.co.uk