On the contrary, we couldn't recognize a miracle unless we had the concept of law, so miracles don't undermine the argument to a lawgiver. In fact as signs , miracles give us additional information about the nature of the lawgiver. Miracles aren't arbitrary or magical as in fairy stories, but always have a clear meaning. Sight to the blind, food for the hungry, healing for the sick, all serve as parables for God's relationship to sinners. Will have uniform evidence against walking on water against one case where it is claimed it happened.
Improbable things happen all the time.
E.g. first time married. Or first time graduated. Or even something no human has done before e.g. climb Everest, find a cure for small pox, land on the Moon Prior evidence is irrelevant once something has happened. But there he was. Walking on water.