Suppose you have a planet covered in a deep layer of supercritical CO2, and animals evolved to live in it.
Discounting the ones who crawl along the bottom, would their method of locomotion be more accurately described as swimming, or flying? Or in other terms, is this layer better described as an ocean, or an atmosphere?
The density of supercritical CO2 is comparable to water--between about 40% and 110%, depending on how high you push the pressure. That seems to argue for "it's an ocean, and stuff will swim in it; animals will be streamlined like fish, use fins, and tend to float". But the viscosity is much lower; it's more viscous than air, but much closer to air than it is to water. That seems to argue for "it's an atmosphere; animals will be aerodynamic like birds, use wings, and fly".
So, which is it? Or is the truth some weird in-between thing?