[In response to the question: What about the fish? I would think that they stayed in water, and weren't taken onto the ark, but sea fish need saltwater, freshwater fish need freshwater, and when the flood came, all the water would have mixed, and become brakish. Very few fish can survive this.]
This can probably be described in many ways, but one way it would work is that salt water is more dense then fresh water, so when they are mixed the salt water sinks to the bottom, and the fresh water stays on top. So when the flood happened, the bottom half of the earth was salt water, and the top half was fresh water. I am sure many fish died, but enough of them survived by staying in their type of water(top or bottom) to reproduce after the flood.