The thing which ISN'T believable, to me at least, is that God might have wiped the solar system over concern about sin, only to have sin back in business as if nothing had happened fifty years later. I give God credit for being several tads brighter than that. I assume the flood was just a cosmic event which we got in the way of.
One explanation is that God made man in His own image, which after the fall for Satan, seemed to be a target for their influence and/or possession. One theory holds that the genetic makeup of the human species had been corrupted by fallen angels to the point that only Noah and His family remained to keep the species lineage. When the judgment came, all was destroyed by waters from within the earth (fountains from the deep) resulting in a planetary deluge for 40 days and nights. After the saturation of the planet by water, erosion and currents shifted about until an equilibrium was achieved and we now have the continents and oceans manifesting that condition.
It also should be noted that prior to the flood, something meteorlogically occurred to allow rain to form, because the droplet size to diffract light in the rainbow is nearly the same size as it takes to form a raindrop. Prior to that time, it appears the partial pressures were different, such that the planet was covered more in a shroud or mist.
Much more science is available to glean if we simply accept His Word as true first, then try to understand what that implies, instead of trying to imply what His Word means from our limited understanding, thereby reading into instead of out of Scripture.