The stunning Volute water feature by Tills Innovations. A vortex being captured and displayed in clarity and detail. What appears to be a solid piece of glass with a spinning vortex. A mesmerising water feature. - My Cool Garden
You can use Rasta blocks made of cement and recycled polystyrene to build a conventional pool with vertical sides. Establish your plant beds around the outside perimeter to naturally filter and clean your pool.
Here's a great example of a conventional pool turned into a natural swimming pond. They offer a much lower maintenance alternative to conventional pools and allow you to swim in clean, chemical free water.
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.