How long would it take him to gather a male and female of each animal? Given that he would have to sail to each continent for it's indiginent species. And bring them to his arc. And feed them. And store them while he sails to the next continent. To Africa. To Antarctica.
It was my understanding from reading Genesis that God sent the animals to Noah. He just had to convince them to get in the ark. One theory I have heard that God put the animals to sleep - like hibernation. And if the whole world was underwater, there would be no need to go to different continents. I figure if he got a couple of basics, then after they got off the ark, they evolved into even more species. But miracles are usually hard to explain anyway.