Why couldn't God drown the Nephilim a.k.a. giants?

In the days of Noah there were Nephilim, the spawn of demons and humans, and also afterward as stated in the Bible. So obviously God couldn't drown the Nephilim. Which stands to reason the demons, a.k.a. Legion, weren't drowned when they tricked Jesus into putting them into a herd of swine and they ran into the sea. Pigs died, but not the demons. What's up with that?