I have no doubt there was a flood event, but taking into account the biblical side of it, the entire physical Earth wouldn't be needed only where humans had lived.
Having travel to Israel, Jordan, etc, I learned some things and had some observations. Israel is the lowest point in the world, the area around the dead sea, and Jerusalem is 1371 feet BELOW sea level. When I was in Jordan, the Wadi Rum desert there is a water line on the tops of all the rock/mountain like formations showing the area had been deep under water at some point in time. It's very likely if humans originated in that general area of the world that it would be completely flooded without it everything. To all human inhabited Earth, all areas humans lived could have been flooded while areas without humans were not.
Recent evidence shows most of the water on Earth did not originate here, came after Earth's formation. Also read hypothesis about Earth potentially having had a ice ring similar to Saturn, all of this could support the flood gates of heaven opening up. (If aside from some event causing localized flooding, if there used to be a lot more dry land and those areas are under sea now).