Signficantly, God did not inform each individual about the coming Flood. Rather, he told Noah who in turn warned his contemporaries of the approaching Deluge
Where in the Bible does it say that Noah warned his contemporaries?
Nowhere in the flood account does it say that Noah warned people of the flood.
The book of Matthew says that when the deluge came people were not knowing (the NWT says "took no note" trying to twist the actual text - look it up in the purple interlinear bible).
Noah was called a "preacher of righteousness" later by Peter or Paul(I can't remember) but that doesn't mean that he "warned" anyone of impending doom. HIs following direction from God and surviving the flood can classify him as a "preacher of righteousness".