15mill gallons (us) of water is around 57,775 m3
to store that much water in a footy (soccer) field sized space you need a tank 100m x 57m x10m
in other words your water tank would take one entire floor of the ark to a depth of 10m The food stuffs I have no doubt would fill that majority of the remaining two floors leaving almost no space for the animals.
Anyway you cut it the ark story (as told in the bible is a prectical impossibility)