Oi.
I'm writing a graphic novel on the premise of a world that has a liquid or semi-liquid atmosphere, separate from a true liquid sea. But since I don't want 'Magical Underwater Adventure!' I've decided (tentatively) on a 'gaseous semi-liquid with non-newtonian-fluid-like physics' - i.e. under low pressure, it takes on more gas-like properties, and vice versa.