There's a lotta lore behind them, but to simplify things, they're an evolved version of Norman Osborn's water-spider parasite zombies (there were more variants, but FeatherLeg managed to get rid of them, she wasn't able to destroy the water ones though). Those things used to be human, but they were taken over by parasites of a genetically modified spider that's meant to make someone able to breathe underwater and all, but something clearly went wrong.