Suppose I could prove to your satisfaction that there is currently not enough water on earth to have caused the great flood. Why should this make you give up creationism? Couldn't God have produced the necessary water to cause the flood and then caused it to disappear when the flood ended? When you can invoke an omnipotent God, no observation constitutes a falsification. That is why creationism is not science.
I think that would be a fairly convincing argument against creationism. I say that, because I've seen no evidence that God, since resting from His work of creation, intervenes in the natural affairs of this world with high drama such as you describe.
You are right in proposing that an omnipotent God provides an easy out when certain facts tend to counter faith. But the proposition of God's creating water for the flood and then taking it away rings more like evolution theories: An ad hoc, unobserved process to justify a desired result.