Stories of global floods (or at least floods of the "known world") are pretty common in ancient cultures. This doesn't prove that there was in fact a global flood, but I do think it tends to downplay the place where they lived being a factor. In other words, everyone talked about a past global flood, not just those in areas where flooding occurred often. Regarding the other thing you mentioned, I would say that many stories in the Old Testament and New Testament are not rare. What is rare is that they are from God and speak to his working among us. Someone being resurrected, someone walking on water, all that stuff appeared in stories before it happened in the Bible. The same might very well be true of Moses in Egypt. There are two good ways of looking at this, I think. First, some of these stories are just going to get told because they make good stories, or spring from the hopes and thoughts of people. Imagination can be a powerful thing. The other way of looking at it (and one that the Fathers endorsed) was the idea that God led various cultures into writing and speaking about certain events, to help prepare them for God's plan for them. In other words, if they had at least some similarities, those who were being converted would better be able to transition into the Church. For example, a greek who had heard about a god resurrecting would have less of a hang up accepting the resurrection of Jesus than someone who had it drilled into him from the beginning that their is no resurrection (e.g., a sadducee). This is why, when Paul got to Mars Hill, he said about seeing the shrine (or whatever it was) to "the Unknown God," Paul wanted to start with something that they already knew. A number of Fathers also said that some of the Old Testament stories predated Greek philosophy and whatnot, and that fellows like Moses had an influence on the rest of the world (though they weren't given credit for it).