In various creation and flood myths the world is said to be remade from water, why would different ancient cultures have this same theme?
Well, if you were to start your own religion, you have to build principles of the, "Cycle of life," so as to explain life and death. To explain this cycle with respect to the world, you think of some ways for large scale destruction. What is something that's almost everywhere in the world? Something that can engulf the world, so as to start a new beginning? The seas, of course!
At least I feel that this line of thought is the reason for most of the myths.