There were prophecies about the flood before Noah came along, but we currently do not have access to them. It's possible that some are in the Vatican Secret Archive, whereas others may still be buried at the same level as everything from that generation.
The Nephilim died before the flood, remember? The 200 Watchers who fathered the Nephilim were condemned to watch their offspring kill each other. The Nephilim probably did not know about the flood (as they killed each other well before it), and I'd bet that the 200 Watchers also did not know about it, as they were cut off from God too early to know about the flood.
>Everyone else was probably as clueless then as we are now.
No. They knew something was wrong, and there were prophets going around telling the truth. Noah was not the only prophet.