This is a challenging question for
Scientific Creationists to answer about the
Noachian flood. While none of the answers fit in with modern day scientific knowledge, some are clever and entertaining.
One is that the atmosphere held much more water vapor before the Noachian flood. This not only explains where the water came from, it also explains why the rainbow predicted by optics didn't appear before the flood! While the atmosphere could not have held all that water under any conditions know to science, at least it sounds good.
On talk.origins I read a detailed refutation of a theory involving floating orbital ice blocks before the flood, but I've never heard the theory itself from a Scientific Creationist, so I'm not sure if the refutation was toungue in cheek.