If the flood wasn’t world-wide and supernatural in origin, then Genesis isn’t true.
the Lord works in mysterious ways...say evolution perhaps. Just like the Pope tells me.
There's some debate among theologians whether the flood was geographically universal or anthropologically universal. It could have submerged all the areas that humankind lived at the time without being global in scope. The Hebrew word used (eretz, I think?) is like English "land", and can refer to the whole earth or a portion thereof.
It’s literally true, but one has to know what it is about.