I think it would be easy to tell where the high water mark of a massive flood like that would have been.
It's interesting that most people think the Bible says the entire earth was covered but evolutionists (not science) claim otherwise. Actually evolutionist academics do commmonly believe the entire earth was covered in water early in its history (see for example https://news.harvard.edu/gazette/story/2021/04/harvard-scientists-determine-early-earth-may-have-been-a-water-world/. But the evidence they cite correlates with Genesis 1:2, not the kataklusmos of Noah's time. So the difference is that evolutionists believe the earth was completely covered in water once, while Genesis records that it was covered twice. Which is funny when you encounter extreme skepticism from people who find the whole idea of the earth being covered outlandish.