A few points for consideration:
1. While I am not aware of any scientific evidence for a
world-wide flood, there is very convincing evidence of a massive regional flood in the Mediterranean Basin. Since this area was heavily populated at the time, and likely the cradle of civilization, this event would have killed large numbers of people and land-dwelling animals. This event is described in more detail in a number of scientific publications, and also at this Christian web site:
http://www.asa3.org/ASA/topics/Bible-Science/PSCF12-97Morton.html
In a nutshell, northward movement and collision of the African Plate with the European plate at one point in the distant past closed off the Strait of Gibraltar, which then allowed the Mediterranean Ocean to basically dry up (more water evaporates from its surface than flows into it from rivers).
As plate movement continued, and downwarping of land at the present-day location of the Straits of Gibraltar occurred, Atlantic Ocean water started to flow back into the Mediterranean Basin. The initial flow would have quickly increased as rapid downcutting occurred, thus creating a massive flood that would have rapidly engulfed the entire Mediterranean Basin, and overwhelmed all living things there.
2. There would have been a time in the Earth's distant past, before plate tectonic activity pushed up crustal material to form mountains, when the Earth would have been much flatter, and floods would therefore have been able to inundate much more land (assuming, of course that ocean water was already present in sufficient quantities to cause flooding). Heck, since about three quarters of the Earth's surface is water, maybe there was a time before mountain building occurred when only a very small portion of the Earth was above sea level, and it would have been much easier to overwhelm the air breathing creatures, including man, that were crowded onto the limited land available.
3. The Biblical flood would only have wiped out land animals- all fish, etc. would presumably have been fine. There appears to be some ambiquity in the Genesis account since it states that the flood wiped out all living things, but doesn't explain whether fish and other sea creatures died. It also doesn't explain why the presumably thousands of fishermen and others that would have been out in the ocean with their boats died, or where the second dove that Noah sent out got the olive branch that it returned with.
I have no doubt about the authenticity of the Bible, and therefore no doubt that God created some sort of massive catastrophic flooding event that wiped out a good part, or maybe even all, of mankind, and a bunch of other air breathing creatures. It is intriguing that the story of a world-wide flood is mentioned in other religions and in the legends of some First Nations people.
However, beyond these two truths, I don't pretend to understand the literal story of the Genesis flood, and have to assume that we are not yet to the point where we can correctly interpret what it says in the Bible about this event.
As little as about 100 years ago, the concept of a massive Mediterranean flood, and the mechanism of plate tectonics that created it, would have been considered preposterous. Now, though our past assumptions have been shattered by newly-discovered knowledge, and what was once considered a myth is now accepted to be reality. Science has had a habit of doing that to us numerous times in the past- forcing us to re-examine our interpretations, but, when a better explanation is established, still keeping the basic tenements of our Faith intact, or even strengthening them. I think we need to learn something from that.
So I view the story of the Biblical flood with interest, but have no doubt that our present interpretation of that event is not likely correct.
It will be interesting one day to have God explain it all to me.