I've found the same thing. Even though they flatten out/get shorter when wet, they still provide three control surfaces to spin the arrow and stabilize it in flight. The only down side is the splash of water drops coming off the feathers as it passes my face.
I have tried CampDry, and the Bohning powder, both worked fine. Never tried the fly floatant, but to me it seems tedious to apply all the past onto feathers. For the past few years I have been using food grade silicone. Works fantastic! I even spray the entire shaft, seems to make the shaft slicker.