On another list, someone asked regarding a blimp animal (hot-air balloon type object) propelling itself by using basically a bellows - and comparison to a nautilus (which bellows water to move around).
"but seeing as air density is very low compared to that of water, you would need huge amounts of air expulsion pressure, so much so that I doubt a biological organism would be able to generate unlike one that lives in water..."
but then ..
"Air density being much lower than water means that the air jet produces less thrust, but also the blimp has to overcome less drag. I'd assume any potential thrust/drag ratio is the same for air as it is for water, since in both cases the same fluids are producing the thrust and drag."
The second comment seems wrong to me.
How do "submarines" compare to "aircraft" in this? They both use propellors to push the fluid around. And for that matter does it make any difference if you're "squirting" the fluid?
Intuitively it seems to me easy for a sea being to move around by squirting water; intuitively it would seem all-but impossible for the Goodyear Blimp to move around by squirting air? (There seems to be "less" of the air; the drag doesn't seem relevant?)