I have been using an "in float" servo to operate the water rudder for 2 seasons in my 1/4 scale Cub, and it works perfectly. Only snag is, I'd have to cut away a section of the float to access the servo, if it needed replacing. I have fitted an extended spindle onto the servo output, which goes up through the top of the float through an O-ring seal. Servo wire is led up float strut into bottom of fuz. through another seal. Servo wire can be easily unplugged so I can remove the floats and fit wheels (normally use the Cub for aero-towing) Servo has never given any trouble so far.