What is fundamentally different between swimming and flying?

Fish are buoyant in water. They have bladders which render their bodies less dense than water. On the other hand birds and planes are not buoyant in air, although hot air balloons and balloon with l.t.a. gases are buoyant in air. read more

That is the main difference between swimming underwater and flying in air. Submarines can become neutrally buoyant using ballast, so they actually fly through the water and they have to apply power to go deeper. read more

Interestingly, the relationship between St and Wa that is implied by Eq. 26 suggests the possibility that the observed tuning of swimming and flying locomotion according to the Strouhal number St (Taylor et al., 2003) may in fact be the consequence of a primary intention to tune for wake vortex ratio Wa. read more