There were floods that covered the world, the problem is that the world was defined as everything between the Euphrates and the Tigris. You don’t need the fantastical to create water, if all you know about the world is a few 100 miles large, everything that upsets your precarious situation is fantastical.
The early humans had experience with snakes, birds of prey and giant cats. Put all the most dangerous things together, sometimes with a human head, and you have the most dangerous creature of all, a dragon that embodies all the worst things you can find in the world, and as cultures shift, those dragon bodies become horse bodies in European, more snake like in Asian or whale and fish bodies in ocean-faring cultures, whatever seems strong and dangerous.