There are many pros and cons to using black plastic for your Water Gardens versus a prefabricated kit.
Using black plastic as a liner for your Water Garden, you will be able to make it as big as you would like. You can make it in a particular shape that you would want and one that would suit your lawn.
You can let your imagination soar when using the black plastic liner to create your Water Garden. You can make it any size or shape you would like. When using black plastic, one downfall, you could get a hole into 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. This would add beauty to your water garden.
When using the prefabricated kits, there are several styles, but you will be restricted to their design and sizes. With the prefabricated kits, these shelves are already made into hard plastic liners. When using the prefabricated kits, you would not have to worry about puncturing a hole into 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 best to work on your lawn and the one that would be the easiest for you to install.