Some of the very best pond liners available come in the form of reinforced geomembrane liners. HDPE's woven blend of polymers is extremely resistant to punctures, tears, and UV radiation. A good example of an ideal HDPE liner would be BTL's AquaArmor. This material can be used to prevent erosion, control seepage, and protect groundwater. It's not manufactured with any toxic materials, meaning that it is completely safe for plants, fish, and humans alike.
BTL's AquaArmor is made out of a woven HDPE core, with a coating of highly durable LDPE or another engineered covering to protect the fibers within. All of BTL's products are rated NSF-61, the highest rating for water safety available, reserved for products appropriate for potable, human-safe applications.