That's the rub. I don't think that those who believe that Jesus was born of a virgin, lived a sinless life, was sacrificed on a cross until dead, was dead for three days and became alive again and went to live with His Father in heaven do disbelieve in a global flood. It's the ones that don't believe all that other stuff that disbelieve that God could literally flood the whole world.
IMO.
I can see where some "young" Christians may be trying to equate the "proof" of science to the Bible. I, at one time, believed that God used Evolution to create the world. But the more you study the Bible and study the origins ideas of evolutionists, you have to make a choice. The two are not compatable.
What helped clear it up for me was the science definition of what a fact is. It is defined as something observable or repeatable in experiments. (That's not a word for word definition, but it's the gist.)
You can observe a fossil. Science has yet observed or repeated in an experiment a lizard turning into a bird, or non-living chemicals turning into life, etc. So when scientists say that "mountains of evidence" proves that so-and-so became such-and-such, it's not a fact.
What they really should say is that our interpretation of the evidence leads us to believe that the most likely natural cause of this is.... That's because science can't measure the supernatural, so it can neither confirm it or deny it.
Sincerely