There are a lot of pros and cons to using black plastic for your Water Gardens versus a prefabricated kit.
With using the black plastic as a liner for your Water Garden, you will be able to make it as big as you would like. You can also make it in a particular shape that you would want and one that would suit your lawn.
When using the prefabricated kits, there are several styles, but you will be restricted to using their design and their sizes. You can let your imagination sore when using the black plastic liner to create your Water Garden. When using the black plastic one downfall would be it is possible that you could get a hole in the liner when installing or when installing your water plants.
You would also have to build your shelves to sit some of your water plants on. With the prefabricated kits, these shelves are already made into the hard plastic liner. When using the prefabricated kits, you would not have to worry so much about puncturing a hole in the liner since it is hard plastic. You would, however, have to be careful and not crack the liner when installing. When using the black plastic or prefabricated liners, you would need to do some research and find the one that is best to work on your lawn and the one that would be the easiest for you to install. You can find a lot of information online and from home a garden center regarding the pros and cons of black plastic and prefabricated kits for water gardens.
Source of Information on the Pros and Cons of Using Plastic over Prefabricated Kits