Whether there has been a flood or not. None of us was there when it happened or not.
If it happened, we may have comfort that all other thingswould come to pass as well. And even more miraculous things than getting a number of families or couples of marsupialians to some distant part of the earth.
The land Nod in which Kain reportedly had gone, was eretz nod, the same word eretz that means earth as well as land in Hebrew. We do not know: was it the whole inhabited land that was meant to be drowned in that flood or the entire earth. We do not even know, whether Moses knew it. He only edited these earlier documents. (His name does not even appear there once.)
What we read is this: If it was as reported, the whole geography and atmosphere must have been very different than it is today. If there is a river (Tigris, Euphrates) or even a land (Cush, Ashur) of the same name today as before, this does not mean much. There is a York and a Orleans and Vienna in the new world, which formerly was and still is in England, in France, in Austria. The New was added just because the old was still there. (Just for Vienna IL it would have been too ridiculous to call it the new one, since nobody knew it anyway).