Waterproofing
Often, hobbyists focus on pond design, varieties of koi, filtration, then waterproofing. I believe it should be the reverse: Invest more on water proofing, filtration, pond design, and koi fishes, in that order. Even when the filter has problems, it can be solved temporarily, using either ready-to-install commercial filters or high-powered air bubblers. But if waterproofing fails, nothing can be done except to reset the whole pond.
Most contractors don't provide a warranty for waterproofing because most are not meant for ponds, but as water-resistant coating for outdoor residential use.
There are two major types of waterproofing use in the industry: the membrane and the liner.
Waterproofing membranes consist of thin layers of watertight material laid over the surface. Tiles can then be added over the membrane. In membranes, major issues revolve around external factors -- floor movements cause small cracks in the pond. Since the membrane method does not contain any mechanical strength, it easily follows the cracking of the pond cement, causing leaks.
Rubber and plastic liners generally resist even floor movements. But because of materials used, they are not durable enough in the long run. They are also often sandwiched by cement, so that repair is not possible without some tearing. Again, this results in resetting the pond.
Then, there's fiberglass waterproofing -- I will be featuring soon. It provides both the mechanical properties of liners, and coating benefits of membranes.