Howdy W.M.,
Even "full scale" float planes use cables, installing a system IN the float is just complicating matters, K.I.S.S.

You not only have to cut a hatch in the float to install a servo tray and servo but you would have to make the hatch removable and waterproof. Wherever the push rod or cables exit the float to get to the rudder, those holes would have to be waterproof as well.

What KIND of floats? Cut a hole (snug fit) close to the rear float strut so you can run the servo leeds up the strut too the fuse, mount the servo in the float and seal all around with GOOO, silicone paste, run a single push rod to the rudder, Y-harness to the air rudder and yer done.

Be sure you use a waterproof servo, they have a seal around the output spline.

I've used a Golden-rod push rod on more than a few floaters connected to the air rudder, no extra servo needed, very simple.