Really how likely would you be to believe it if somebody came around and said there is going to be some kind of worldwide catastrophe which you have never seen before but you better follow me or you'll die. I think most people in any age would be skeptical.
Woudn't it have been better for God to have appeared in the sky and told the world listen to Noah and follow him or your all going to die, then I think Noah would have had plenty of believers join him on the ark.
This idea of faith in things unseen troubles me because people are more likely to be misled by following somebody based on faith and arrive in a place like Jonestown as they are to get it right and make it to the ark.