If you are an all powerful god, why use a global flood anyway? If you are omniscient you will know that you will regret using a flood to murder everyone.
If god is powerful enough to dissipate all of the energy released in the flood so that 8 humans and some animals could live. If he could keep the water salty enough so some fish could live. If he could teleport animals to other parts of the earth to start new evolutionary lines. If he was powerful enough to cover up all the evidence for a global flood, why not just zap the folk who he wanted out of the way?
I dunno, perhaps he was just too cowardly to do the dirty work directly so wanted to use the forces of nature?