Here's a couple of photos of two ways that I have dealt with the water rudder issue. One is in the float - and I have never used a 'waterproof' servo and never had a problem - and the other is using an externally mounted servo on the fuselage and this is connected to the 'air' rudder via a control rod and also - when the plane is on floats - to the water rudder via a Sullivan Gold 'N Rod. Both have worked very well for me.