When you are building a water garden or pond and will be using a liner or membrane choose a PVC or high-quality EPDM membrane. While PVC is relatively cheap and available in thickness from 0.5 mm to 1 mm the average lifespan of this membrane is only around 15 years and another downside is that PVC is more rigid rigid and requires therefore a good dose of know-how to be able to place it correctly.
EPDM is very flexible and stretchable. This membrane is more expensive than PVC, but is often sold with a warranty of 30 or even 40 years. EPDM thickness varies between 0.75 mm and 2 mm. Generally, it is estimated that 1.15 mm thick enough, without requiring additional protective sheeting. Still ask for advice in the store. For some ponds to accommodate carp koi and large fish, recommended a membrane in EPDM. For a pond for plants only, or a small pond with plants and fish, PVC is suitable.
Using a premolded or pre-formed basin simplifies the pond provcess to an extent. The hole can be roughed in with less precision and the molded plastic basin can be dropped and levelled fairly quickly. Many basins have planting shelves to set your aquatic plants and they are relatively strong and withstand abuse and won't puncture easily. Making sure they are level is a key when setting a preformed pond into your yard!