Purely to contribute to the discussion about the different Bible translation's interpretations:
People didn’t realize what was going to happen until the flood came and swept them all away. NLT
and they knew nothing about what would happen until the flood came and took them all away. NIV
And knew not until the flood came, and took them all away; KJV
and they did not understand until the flood came and took them all away; NASB
Devil's advocate: how could the people suddenly understand/realize/know what was happening at the commencement of the flood, if they had no prior conception or information about it?
They clearly had no faith/belief that such an unbelievable event would take place - despite the fact that the old man and his family were "putting their money where their mouths were" by building a huge wooden structure to float to safety.
Also, Jesus was making a comparison with the warning which he and his followers would issue to the Jewish nation regarding their impending demise and that of Noah's time.
The wording/translation quoted in some versions appear to be over-engineered - for whatever reason.
2 Peter 2:5 says that "Noah was a preacher of righteousness", so at the very least he was telling his contemporaries something which was unappealing.