Chapter 6 opens with a description of the lawless violence that has engulfed the world. There's also a reference to the "Nephilim", whose exact identity remains a mystery.
One thing that's striking about the Flood narrative is the amount of detail, in the measurements of the Ark and especially in the chronology of the Flood itself. (And we have to assume that the information in the text represents Divine revelation, as it must have been impossible for Noah to keep track of months and days when the sky was obscured.)
Incidentally, the word for the Flood used in the text, [mabul], doesn't mean simply a flood of water, it means destruction.
I want to zoom in on verse 2. "The sons of the mighty [bnei ha-elohim] saw the daughters of man ..." Now this is tricky to translate, because "elohim" can mean either "God" or "mighty ones", but it's clearly used in the second sense here.
Also notice that this verse very subtly echoes the refrain of 1:4, etc., "... and God [Elohim] saw that it was good," only here of course it's in a negative sense.
Now the next part of the verse is often translated in English as "... [they] saw that the daughters of man were fair (or, "beutiful", etc.)", but the word that's actually used [tovoth] simply means "good". And in fact that's how the Artscroll edition renders it, and I think it's the most straightforward understanding of the verse. The daughters of man may have been beautiful or not, but they carried themselves with decency and dignity - they were *morally* good.
And it was this goodness that the corrupt, powerful men saw in them - and they despised them for it.