seeing the the addition of actually cool water I think simple planes 2 needs to have one super important new part, the mass changing fuselage. What this part would do is be able to change its mass to either positive or negative numbers while in game, for example if you built a submarine you could use these blocks to change the depth at which you are at. they would basically serve as ballast tanks. or you could build a lighter then air airship that could use these blocks to float.

if this is added one thing that would also need to be added is for water to get denser the deeper it is, and air to get less dense the higher you go . what this would do is make wings have higher authority in denser areas and less authority in less dense areas. it would also cause the mass changing part to cause it's craft to have a neutral buoyancy at an elevation. this would also make fins and rudders underwater work better.