First off, who are the Nephilim of Genesis 6:4 and Numbers 13:33? Evidently, they are a race of misbegotten superhumans, born of unholy unions between mortal women and fallen angels. At this point, the average Bible reader may become uncomfortable on account of the obvious parallels to pagan myths of demigods, but it gets so much weirder if one strays outside of the usual canonical scriptures.
At any rate, the question for the apologist here is how this mythological race of preternatural giants survived the great flood, in which "All in whose nostrils was the breath of life, of all that was in the dry land, died." I have yet to see a passable reply to this problem.