Natural pools do best when lined with strong, flexible, NSF certified geomembranes. These ensure that your water doesn't seep into the surrounding soil, and that no chemicals are leached into the pool due to toxic materials in the liner. A geomembrane liner is strong enough to support plants and growing media, and will be comfortable for your swimmers to brush up against.
Concrete is typically a poor choice in natural swimming pools. Concrete is expensive to install and maintain, and is not certified for potable water use. Fiberglass is similarly uncertified, and can be especially susceptible to warping or UV degradation.
