If the Flood were merely a story (or a local occurance) I would suppose that Jesus would still refer to it, much as we might refer to the Good Samaritan story in order to make a very important point about being a true neighbor.
Whether there was an actual good Samaritan or not is kind of besides the point - it's the story that resonates. Just because Jesus made reference to the Flood does not in itself make the Flood a historical occurance, nor does it make it global. Could the Flood merely be a story, which Jesus pointed to in order to make a point?