my personal experience with plastic control rods has been dissappointing....a bend in the run increases the friction enormously.....getting water in them increases the friction even more. End result was little if any throw on the water rudders. The ultimate system is pull pull cables like the full size use.

A servo in the float with a straight run would be very effective too. The traxas 2080 servo should be enough for the water rudder. The lowest cost Hitec waterproof servo puts out over 100 oz inches of torque (a bit of overkill) and is nearly twice the price of the traxxas 2080. If you want a full size servo, the traxxas 2056 is about 3 bucks more....