It is physically impossible for water not to be flowing in through the open nostrils of Red when she is underwater, thereby drowning Jacob and Maisie who are sitting right behind it. However, if you watch carefully when Red surfaces, you'll see a red edge of skin shift across the nostril before Maisie and Jacob get blasted with air. That appears to be a nictating membrane of some sort, albeit clear so people can see through it underwater.