The deeper the pond, the warmer it will be in the winter in the deep end for the fish to hibernate in. If the pond is too shallow for the fish in question it will put a lot of strain on the fishes health as shallow ponds are colder and more vunerable to temp extremes and large fluctuations in temp etc, and the fish will come out of hibernation in spring feeling weak and stressed, which is not good as in this state the fish can easily fall ill to diseases and parasites etc.
Also koi kept in ponds to shallow tend not to live as half as long as they should and often suffer from stunted growth. Your pond really isn't anywhere deep enough for fish which can grow to 3ft long, i would definately not advise koi for your pond unless you make it a lot deeper.