You seem to be asking why a hydrofoil is not penalized by nature for a fine entry, where an airfoil is most efficient with a blunter one.



I believe (corrections welcome) that the shape of an airfoil is designed to avoid turbulence, most often the product of the airstream 'detaching' from the airfoil. Naturally airfoils differ in purpose and so in design, but for a given foil, the 'goal' is to 'hide' as much of your cross section in your given length without inducing turbulence.Detachment is hardest to prevent at the exit, or low-pressure end. So, for maximum efficiency, you put the thickest cross section forward of the center, providing a finer exit than your entry.



Now, air is compressible, which gives you lots of freedom to make the entry blunter than if air weren't compressible - the "bow shock" or bow wave mentioned above is more forgiving in a compressible medium. The "quicker" you get to max cross section without creating turbulance, the finer your exit cdan be. Hence, the more draft you can put in the given length, or the more efficiently you can get your target draft through the air w/out turbulence. So you go as blunt as you can get away with without inducing turbulence.



H2O isn't compressible, at the kinds of forces in play at the surface of the ocean, anyway. So you have less freedom to do a blunt entry, and correspondingly more reward for a fine entry. All the factors about exit and detatchment still apply, you just don't have the freedom to go as blunt in water.



Hope this helps!