Geelong Nullification

Swimming creatures cannot swim through the air above landmasses like they can through the Songlines. The leading theory is that the land itself absorbs enough of the latent magic in the air to put into keeping itself aloft that the ambient level drops to the point it cannot sustain Skyswimmer's ensorglid organs. Creatures that adapted to fly on landmasses however can still gracefully traverse the songlines, likely because it is even less effort than flight requires.