Actually, the cubit used was probably somewhat larger than 18", more like the 21" egyptian cubit, and the boat likely had many decks. The Babylonians report 6 decks in the Epic of Gilgamesh. That would be 6.3 acres of space.
How much space does it take to pen up Zoo animals for the winter? Not that much. Our Philadelphia Zoo is just 42 acres total, and most of that space is walking paths, water, summer grazing habitats, and the like, not animal enclosures. The non-hibernating animals would have required some food of course, but again, 6 acres is a lot of space for pens and food stores, and some of the food would have been produced by other animal's natural reproduction (i.e. bird eggs, fast-breeding mice and rats and rabbits, while fish could of course be caught off the side or back of the boat, etc.).
I think the problem is exaggerated.
"you shall make it with lower, second, and third decks. - Genesis 6:16
One man who has claimed to have seen the ark broken in half said he saw ovens and cages inside. I thought that was pretty interesting.
How many keepers does the Philadelphia Zoo need to look after the few-hundred resident species? How many keepers would it need if *everything* had to be done by hand? No fresh water in pipes. No automatic removal of waste. No trucks to drive food around. No vetinerary servies on call. No artificial lighting. Just the 20,000,000 species aboard (try packing that into 6 acres, or even 600 acres. IIRC the bible specifies 3 decks not 6. And your space figure doesn't allow for storage space for fresh food and water, and for the internal bracing in such a craft that would probably require about 30% of the internal volume. The 8 people would get a few seconds per species in the entire year to look after them. Numerous species have special dietary requirements, temperature requirements, humidity requirements... And since you brought fish up did Noah carry the freshwater fish on the ark or the saltwater fish? He had to carry one or the other. Some of the saltwater fish require enormous pressures to survive, so maybe the flood was salty and Noah carried the freshwater fish, but in that case how long would it have been until the land was fertile after the water receded? Why did God hide all the physical evidence that this event ever happened, yet permit a book describing it to continue to exist? Once the animals landed every predation event for the first few months would represent an extinction, and carnivores need lots of prey to eat. The list of rational objections just goes on and on; all I've done there is summarise a small number of the most obvious. And all this so that God (an infinitely powerful being) could kill everyone-8 in the world in a moronic way that required millions of miracles, a way that just happens to match exactly the myths that you'd expect early hydraulic civilisations to tell. How can people take this stuff seriously?