Well, Noah's Ark isn't the only ancient 400'+ wooden ship in history. The chinese were making them in the 1400's and there is the Greek reference above from wikipedia, and many others.
Also:
In the book Noah’s Ark: A Feasibility Study 4 , creationist researcher John Woodmorappe suggests that, at most, 16,000 animals were all that were needed to preserve the created kinds that God brought into the Ark.
The Ark did not need to carry every kind of animal—nor did God command it. It carried only air-breathing, land-dwelling animals, creeping things, and winged animals such as birds. Aquatic life (fish, whales, etc.) and many amphibious creatures could have survived in sufficient numbers outside the Ark. This cuts down significantly the total number of animals that needed to be on board.
Another factor which greatly reduces the space requirements is the fact that the tremendous variety in species we see today did not exist in the days of Noah. Only the parent “kinds” of these species were required to be on board in order to repopulate the earth. 5 For example, only two dogs were needed to give rise to all the dog species that exist today.
Creationist estimates for the maximum number of animals that would have been necessary to come on board the Ark have ranged from a few thousand to 35,000, but they may be as few as two thousand if the biblical kind is approximately the same as the modern family classification.
A little over half of the Ark would have been needed for the animals.
As far as the salt vs. fresh water fish, here's an interesting commentary:
But what about the fish and other marine creatures? Obviously, they weren't taken on board the Ark. How could they survive, particularly both fresh and salt water forms? As a matter of fact, most of them didn't survive. Over 95 percent of all fossils are marine creatures. They died, and are fossilized, by the trillions. Many are buried in great fossil graveyards, tightly packed together, choked with sediments, buried before they had time to decay. Obviously, they didn't live in the environment in which they died. But how could any have survived?
In the complex of events and conditions that made up the Flood, certainly there were pockets of fresh and/or clean water at any one time. Remember, it was raining in torrents, and we can expect that the rain was fairly fresh water. Many studies have shown that waters of various temperatures, chemistries, and sediment loads do not tend to mix; they tend to remain segregated into zones. It would be unlikely for any one area to retain such zones for very long during the tumult of the Flood, but on a worldwide scale, some such segregated zones would have existed at any given time. Furthermore, we don't know the tolerance levels of pre-Flood fish for sediment, salt, and temperature. Modern fish have a great variety of responses to different environments. Perhaps before the Flood, fish were even more adaptable.
