Its also typical to need ot add a bit of a "sub-fin" to an airplane when you put the floats on. (calling them pontoons is fine too...)

The floats add side area that is ahead of the nose of the aircraft... you need to aerodynamically balance this. Most trainers have a bit more fin/rudder area than necessary, but to maintain the same yaw characteristics you'll need more. A section of fin added under the fuselage appx 10% to 20% the area of the fin/rudder will help.