You can take a look at this article. I think the most persuasive arguments include "How did Noah get the animals to the ark?" and "How did Noah care for the animals on the boat?" In addition, it's worth contemplating how the hell kangaroos were supposed to walk and swim all the way up to Australia without leaving a single trace of evidence.
If after all this someone still wants to believe in the flood, he would have to take it blindly on faith. The only answer a Christian could offer is that it was a miracle because without a miracle, it's simply impossible for the flood to be real.