A global flood defies science. This story was my first step away from bible literalism.
Beyond the geological implications, the diversity of life that we now have could have not came from the small sampling brought onto the ark.
Interestingly enough, most people who believe in a global flood fervently disbelieve evolution. Yet extremely fast evolution could be the only possible way such a diversity of life could develop. We're talking much quicker evolution than biologists propose.
Here's a short article that goes into detail, from someone with far more scientific knowledge than I:
http://paleo.cc/ce/ark-gene.htm
As ProdigalSon pointed out there were many other myths about a global flood. And as others pointed out there was possibly a local flood. What I find likely is that there was a disastrous local flood, possibly more than one in different areas. Through oral tradition these cultures embellished and mythologized it.
Nicolaou made an insightful point on faith being a hinderance to understanding. I learned this firsthand while discussing evolution with my dad. No matter how much convincing evidence I presented to him, he threw out or distorted anything that conflicted with his worldview. I realized that while religious people consider their faith a virtue, in reality it often hinders their reasoning.