Omnipotent god created the world in 6 days, and being omniscient he knew exactly how things would pan out for his creation. In spite of that, he gave Adam and Eve free-will and thus gave them the impression that they could only blame themselves for the terrible things that happened in the world. Since in those times even the wisest people on earth knew less about science than an average primary school child knows today they had no option but to invent stories of creator gods and as the stories were told time and time again they became more powerful. How did they know that the stories were true? They just did. Everyone knew it.
Let's assume for a moment that there was really a god behind the creation (even though the logic does not support the idea if you think about it carefully, as somehow the creator must have been created). This all-powerful creator of the world somehow made an error in his plans and humanity lost its way and started worshiping other (even more false) gods. (See this previous mini-series, A family of Old Testament gods? for other thoughts on that). People forgot to be subservient to the god who had revealed to them that he had made them. They got worse and worse and the creator did nothing to nip things in the bud, even though clearly he could have found a way to create a few natural disasters to frighten them back onto the right track.
No - foresight having failed him - this loving creator eventually could see no other way out of the problem than to wipe out most of creation and start again. Presumably, having put a lot of effort into creating the world he did not want to start again with the whole day and night business, and he skipped the creation of the land and the seas. I suppose they were good enough to recycle, whereas (Jesus not having been invented yet) the redemption of humanity was beyond the imagination.
On the other hand you could speculate that this loving god actually intended things to happen this way.
Or thirdly - maybe there wasn't a loving god at all and the flood myths are just a vestige of a local disaster which wiped out a few villages or perhaps a city somewhere near the home of the originator of the story, and you know how stories get amplified and exaggerations creep in over the weeks, months, years and ages. (Incidentally this idea of a localised deluge is more in tune with the version of the story in the qu'ran, and indeed of the Epic of Gilgamesh. More on those later.)
So assuming that there is some truth in the flood myths, the conclusions can only be that
- god was not there;
- god was careless enough to allow things to get to a state where he needed to send a flood;
- god deliberately and malevolently set things up so that it was inevitable that he would destroy most of mankind.
I will leave you with one further thought from my online friend @RosaRubicondior who tweeted:
"If we're all descended from Noah, what are the odds that only one group of descendants remembered the Ark, the flood & Noah's name?"