Or, if you make it an electric you can mix differential throttle using two seperate speed controllers, Oh I see you need THREE!

Unless you are intent on some super accurate scale model I would just take the simple route and rig a water rudder, right where the hull starts that sharp upsweep. Russ Farris

P.S. Jetfxr, unless it is dead calm a seaplane will weathervane at taxi speeds and you need either differential power or a water rudder.