The alternative theory to Noah and the Ark is that it never happened. It's just a story that just isn't true. 4000 years ago people believed the world was flat and that the sky and the clouds went up forever. They didn't know how high mountains were and how much water it took to cover them. They thought that a big flood could cover the Earth and the water would evaporate or run off somewhere. They were wrong.

The alternative theory is that it's just a story and has no more basis in truth than any other fairy tale. No reasonable person could believe this is true.