An alternative to chlorine pools, the BioTop Natural Pools use plants to keep water clean and clear. Wikipedia has some more info on natural swimming pools as well. These could make a great addition to a permaculture design site.
Eco-Friendly Natural Swimming Pools Natural pools eliminate harsh chemicals like chlorine by combining the cleaning properties of plants with bio-filtration and skimming systems. The pools are separated into two zones the plant zone and the swimming zone.