You're using secular reasoning to try to understand a miracle.
Either God exists or He doesn't. I believe that He does. If someone believes otherwise, that doesn't bother me in the least. I'm not going to lecture someone and tell them that they need to share my faith, or anything like that.
That being said, I've never seen to logic in the type of argument you're presenting. I've even seen people here who profess to believe in God make similar arguments. The arguments are that the Biblical flood couldn't have happened because various aspects of it are too hard to believe. Such as the ones you've mentioned. How could fresh water fish survive in salt water? How could tropical animals have found their way back to their native lands so soon?
But **IF** God exists, those things aren't problems in the least, are they? Now, you may say you don't believe in God (I don't know if you do or not, feel free to post your opinion on this if you feel comfortable discussing this). If you don't believe in God, then we simply have two incompatible world views. You're entitled to believe in a purely materialist worldview, and I'm entitled to believe that there's a realm beyond the material.
But if you do believe in God, how do place limitations on Him? How could you maintain the following positions coherently?
A) I believe in God.
B) But I don't believe He could have pulled off that Noah's flood deal. The salt water fish would've died. The tropical animals couldn't have found their way back to the equatorial jungles afterward. Etcetera.
But if God exists, and is indeed God, He could solve any of those problems with the proverbial snap of a finger, couldn't He?
You can't win an argument by explaining away every thing as a miracle. Its a cop out.