In seeking to understand what is going on in the account of the flood and why the waters that came upon the earth, we must understand the ANE cosmology. The waters of the flood cause the world to revert the the chaos of a pre-creation state. The flood reveals cosmic warfare between God and Yamm, the god of chaos and storms.
Can a Christian be demon-possessed? Scripture indicates that a Christian cannot be demon-possessed, but this does not mean there is nothing to be wary of. Not all who say they are Christians truly are, so maybe those who are Christian in name only can be possessed. Furthermore, Christians can still be oppressed and influenced by demons.