Never mind that we are still puzzled about how the chains were forged to begin with. Someone had to have swam into the lake in order to secure the chains to his body. In fact, there must have been at least 4 zombie scuba divers who plunged into the icy water, attaching four different chains. How else would they have been able to grab hold of Viserion? I highly, highly doubt that they just tossed the chains in the water like they were fly fishing, hoping that the chains would catch onto the corpse.