[usrusr]

> 2) A plethora of objects to smack into

On the ocean? I'd say that not running into objects that raise high from an ocean would rank pretty low on the scale of control problems that are hard in 2020.

"up to 500 km/h" are military devices built for maximum speed at minimal distance from the surface. A short distance people mover aircraft that tries to trade speed for energy conservation would have very different numbers. And it would be far more nimble, by being able to quickly raise out of the ground effect level, than swimming ocean-surface vessels that are already quite capable of not hitting objects on the ocean (you'd definitely want to improve on the state of that art though).

The biggest issue with this concept, outside of the clearly problematic cost/market size ratio (because designing planes is never cheap) would surely be that an aircraft traveling in ground effect lacks the altitude energy store that allows conventional aircraft to deal surprisingly well with engine outages (or sensor issues) outside of short time windows during start and landing. You'd probably need to spend a lot of mass and effort on making the fuselage properly boatable in emergencies even if it's never intended to swim more than once.